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Do capital breeders like Atlantic herring (Clupea harengus) exhibit sensitive periods of nutritional control on ovary development and fecundity regulation? James Kennedy, Jon Egil Skjæraasen, Richard D.M. Nash, Anders Thorsen, Aril Slotte, Tom Hansen, and Olav S. Kjesbu

Abstract: A laboratory study was undertaken to investigate whether Northeast Atlantic herring (Clupea harengus), i.e., Norwegian spring-spawning herring, exhibit a ‘‘sensitive period’’ during the feeding season in which ovary development is particularly susceptible to food availability and (or) energy reserves. Groups of herring received similar amounts of food over the restricted summer – early autumn feeding season but the food availability was varied temporally between groups. The herring, an extreme capital breeder, did not exhibit a sensitive period, as there was no difference in fecundity or ovary maturation between groups. However, individuals that did not reach a Fulton’s condition factor (K) above 0.70 during the feeding season were less likely to begin ovary maturation. Those below this threshold showing ovary development began later and had a higher intensity of atresia than fish in better condition. To maximize fecundity, females recruited significantly more oocytes than they could support through to spawning, thus the oocytes were subsequently down-regulated. Some would have skipped spawning in the coming spawning season; these fish had a very low K. Taken together, this study demonstrates that this capital breeder has developed a suite of reproductive strategies to synchronize the production of the highest number of eggs energetically possible. Re´sume´ : Nous avons entrepris une e´tude de laboratoire pour examiner si les harengs (Clupea harengus) du nord-est de l’Atlantique, c’est-a`-dire les harengs norve´giens a` fraie printanie`re, posse`dent « une pe´riode sensible » durant la saison d’alimentation pendant laquelle le de´veloppement de l’ovaire est particulie`rement influence´ par la disponibilite´ de la nourriture et (ou) les re´serves eńerge´tiques. Des groupes de harengs ont reçu des quantite´s semblables de nourriture pendant la courte saison d’alimentation de l’e´te´ et du de´but de l’automne, mais avec des variations temporelles de disponibilite´ de la nourriture entre les groupes. Le hareng, qui se reproduit de façon tre`s marqueé a` partir de ses re´serves, ne posse`de pas de pe´riode sensible puisqu’il n’y a pas de diffe´rence de fećondite´ ni de maturation ovarienne entre les groupes. Cependant, les individus qui n’atteignent pas un facteur de condition de Fulton (K) supe´rieur a` 0,70 durant la saison d’alimentation sont moins susceptibles de commencer leur maturation ovarienne. Les poissons sous ce seuil qui affichent un de´veloppement ovarien de´butent plus tard et subissent un taux plus e´leve´ d’atre´sie que les poissons en meilleure condition. Afin de maximiser leur fećondite´, les femelles produisent significativement plus d’oocytes qu’elles ne peuvent rendre jusqu’a` la fraie; les oocytes sont donc subse´quemment re´duits par controˆle descendant. Certains poissons qui ont un K tre`s faible auraient omis la fraie durant la saison suivante de reproduction. Dans leur ensemble, nos re´sultats montrent que ce poisson qui se reproduit a` partir de ses re´serves a de´veloppe´ une se´rie de strate´gies reproductives afin de synchroniser la production du plus grand nombre possible d’œufs compte tenu de l’eńergie disponible. [Traduit par la Re´daction]

Introduction Fish fecundity is known to vary over temporal (Kjesbu et al. 1998; Kennedy et al. 2007) and spatial (Witthames and

Greer Walker 1995; Morgan and Rideout 2008) scales. In many cases, these variations have been linked to food availability, which affects energy reserves, both at the individual level (Bagenal 1969; Kjesbu et al. 1991; Kennedy et al.

Received 12 January 2009. Accepted 27 August 2009. Published on the NRC Research Press Web site at cjfas.nrc.ca on 11 December 2009. J20987 Paper handled by Associate Editor C. Tara Marshall. J. Kennedy1,2 and J.E. Skjæraasen. Department of Biology, University of Bergen, P.O. Box 7800, 5020 Bergen, Norway. R.D.M. Nash, A. Thorsen, A. Slotte, and O.S. Kjesbu. Institute of Marine Research, P.O. Box 1870 Nordnes, 5817 Bergen, Norway. T. Hansen. Institute of Marine Research, Matre Research Station, 5984 Matredal, Norway. 1Corresponding 2Present

author (e-mail: [email protected]). ˚ lesund, Norway. address: Møreforsking Marin, P.O. Box 5075, 6021 A

Can. J. Fish. Aquat. Sci. 67: 16–27 (2010)

doi:10.1139/F09-159

Published by NRC Research Press

Kennedy et al.

2008) and at the stock level (Bagenal 1966; Kjesbu et al. 1998). Many of the studies on fish fecundity have looked mainly at changes of fecundity between years (Kjesbu et al. 1998; Gundersen et al. 2000), but some recent studies have shown that fecundity within a year is not constant and that fish will reduce the number of developing oocytes by atresia as ovary development proceeds (down-regulation) (Kurita et al. 2003; Kennedy et al. 2007; Witthames et al. 2009). The percentage of oocytes lost during down-regulation has been seen to be as much as 56% in herring (Clupea harengus) between July and March (Kurita et al. 2003) and up to 50% in plaice (Pleuronectes platessa) between September and February (Kennedy et al. 2007). This down-regulation is considered to be a method by which fish regulate their fecundity to match their available energy reserves (Kurita et al. 2003). Within the yearly maturation cycle, there are known to be some energy-sensitive periods in which food availability and (or) energy reserves can affect gonad development. Burton (1994) demonstrated that winter flounder (Pseudopleuronectes americanus) that were not fed during the early part of the feeding season (and in low condition) were likely to be non-reproductive in the coming reproductive season. Turbot (Scophthalmus maximus) that are offered lower food rations during oocyte recruitment have a lower chance of maturing (Bromley et al. 2000). Similarly, plaice that show a low growth brought about by low surplus production during the summer feeding period are likely to skip spawning (Rijnsdorp 1990). Total body weight about 3–4 months before spawning, during early vitellogenesis, has been shown to be the best proxy for fecundity in Atlantic cod (Gadus morhua) (Skjæraasen et al. 2006). To date, no study has examined if such a sensitive period exists for a pelagic, plankton-feeding fish such as herring. It is known that food levels are important in the determination of fecundity in herring. A reduced level of feeding in a tank experiment on captive local Bergen herring resulted in a decrease in potential fecundity and a greater intensity of atresia, but there was no effect on relative potential fecundity (oocytes per gram of body weight) or oocyte size (Ma et al. 1998). A complementary study by Gonza´lezVasallo (2006), using a much greater range of feeding regimes, also showed that potential fecundity, but not relative potential fecundity, was affected by food level, i.e., differences in potential fecundity were the result of differences in the weight of the fish. Hay and Brett (1988) tested the effect of feeding or non-feeding in the last three months before spawning on captive Pacific herring (Clupea pallasi). This led to the fed fish having a lower weight-specific fecundity than unfed fish due to a lower loss of somatic tissue, but there was no difference in the length-specific fecundity between treatments. They thus concluded that feeding at this time had no effect on the regulation of fecundity. The three studies mentioned above are basically in line with the surplus-energy model of Rijnsdorp (1990) stating that relative fecundity reaches a stable plateau for fish in good condition. However, this may not be so simple, as fecundity is affected by atresia during large periods of the ovary maturation and the intensity of atresia is not constant through maturation; however, its effects on fecundity must be considered to get accurate estimates of realised fecundity

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(Ma et al. 1998; Thorsen et al. 2006). Also, it has been found that in extreme situations, herring skip spawning. Two herring in extremely low condition (Fulton’s condition factor (K) < 0.55) were found to have 100% atresia and so skipped spawning during a laboratory experiment ´ skarsson et al. (2002) also found (Gonza´lez-Vasallo 2006). O a low number of herring that skipped spawning in the field with K of < 0.70. It has been suggested, however, that up to one in two herring may skip their second spawning season (Engelhard and Heino 2005). This has been inferred from scale readings taken from herring collected on the spawning grounds between 1935 and 1973 that showed a significantly lower number of second-time spawners compared with thirdtime spawners. The proportion skipping spawning has also been linked to the size and condition of the herring as firsttime spawners and to climatic factors (Engelhard and Heino 2006). The reasons for the discrepancy in numbers of skipped spawning in herring between these studies may be ´ skarsson et al. (2002) sampling herring from the due to O overwintering and spawning grounds, whereas the study by Engelhard and Heino (2005) is based on these herring not being present on the spawning grounds and therefore they ´ skarsson et al. (2002). would not have been found by O The Norwegian spring-spawning (NSS) herring stock is currently the largest herring stock in the world, with a spawning stock biomass (SSB) of approximately 12 million tonnes in 2006, and supports a large commercial fishery (International Council for the Exploration of the Sea (ICES) 2007). The copepod Calanus finmarchicus typically dominates the diet of NSS herring, particularly during the early phase of the annual feeding migration, but the herring also feed on a variety of other zooplankton prey (Holst et al. 1997; Dalpadado et al. 2000; Prokopchuk and Sentyabov 2006). Zooplankton has a patchy distribution across the Norwegian Sea and the distribution varies between years (Olsen et al. 2007). The change in plankton distribution is usually due to changes in environmental conditions. Because of the conservative nature of herring migrations (Corten 2002), herring have been known to migrate to areas where feeding was good in previous years only to find an absence of zooplankton (Jakobsson 1969). If herring have a period during their ovary development that is particularly sensitive to food or energy levels, then if such an event as described in Jakobsson (1969) occurred, it could have a strong effect on fecundity and maturation of herring. Herring generally feed from about April until about September; between September and the end of spawning, herring will live off the reserves built up during the feeding season (Slotte 1999). Gonad growth begins in May (Kurita et al. 2003) and most of the growth is funded from stored reserves (Slotte 1999). True vitellogenesis begins between June and August, with 100% of maturing fish having started by October (Kurita et al. 2003). Down-regulation is carried out on vitellogenic oocytes throughout the maturation cycle but is particularly prevalent during October–November (Kurita et al. 2003). However, atresia is absent in fish with an average oocyte diameter of greater than 1050–1200 mm ´ skarsson et al. 2002; Kurita et al. 2003; Gonza´lez-Vasallo (O 2006). For herring with a K greater than 0.70 or close to spawning, the variance in fecundity is largely explained by ´ skarsson et al. 2002; Kurita et al. 2003; O ´ skarsson weight (O Published by NRC Research Press

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Materials and methods Experiment 1 NSS herring were caught on spawning grounds off the Norwegian coast (Fig. 1) in April 2006 and transported to the Institute of Marine Research (IMR), Matre Research Station, Norway. The fish were then distributed randomly among four groups and housed in four 5 m diameter circular tanks with a water depth of 2 m. A constant flow of seawater at ambient salinity and temperature (8–14 8C) was provided. The fish were acclimatised to the tanks for two months before the start of the experiment. During this time, the fish were fed to satiation with krill and gradually weaned onto salmon pellets (Skretting Nutra Olympic, 3 mm). All injured and dead fish were removed daily. At the commencement of the experiment on 15 June 2006, there were approximately 200 fish in each tank. Each herring group was subjected to different feeding regimes (Table 1a). On each feeding occasion, the fish were fed to satiation. Feeding ceased on 15 October, which is the approximate end of the feeding season for NSS herring (Slotte 1999). The experiment ran until 5 March 2007. Thirty fish in each group were sampled in April (upon capture), June, August, October, and December 2006 and March 2007. Each fish sampled was measured for length (nearest 0.5 cm), weight (nearest 1 g), and the weight of the intestines and the gonads (nearest 0.01 g). A gonad sample was stored in 3.7% buffered formaldehyde for estimation of fecundity. Scales were also taken for age reading. A fat reading of the fish was taken during the December and March sampling using a fat meter (model FM 692, www.Distell.com) (this was calibrated by the manufacturer for herring to give a fat content of the entire fish). This was done by taking a reading above and below the lateral line in the midpoint of the length axis of the fish and calculating the average of the two readings. Experiment 2 Herring were caught close to Øygarden (approximately 50 km north of Bergen), Norway (Fig. 1), by a local fisherman in May 2007 and transported to IMR, Matre Research Station, Norway. The fish were then distributed randomly among three groups and housed in tanks of sizes similar to the ones used in the previous year (see above). The experiment ran from 11 July 2007 until 25 February 2008. The fish were fed krill pellets from capture until the start of the experiment and then fed Calanus pellets (produced by No-

Fig. 1. Map showing the location in Norway of the capture positions of the herring for experiments 1 and 2 (shaded circles). Inset shows location in northern Europe. 70.00

1

65.00

Latitude (°N)

and Taggart 2006). This is also common for other species of fish (Koops et al. 2004). This study examines if there is a sensitive period, during the feeding season, when food must be in plentiful supply for the successful maturation of gonads in NSS herring. This was done through an experimental study in which different groups of herring received the same total amount of food over the feeding season, but the rations given varied temporally between groups. This allowed us to examine (i) the link between individual energy reserves and fecundity and oocyte growth and (ii) the likelihood of herring skipping spawning.

Can. J. Fish. Aquat. Sci. Vol. 67, 2010

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60.00

Norway

Sweden

200 km

55.00 0.00

5.00

10.00

15.00

20.00

Longitude (°E)

fima, Bergen, Norway). Each group was subjected to a separate feeding regime (Table 1b). Using the results from Gonza´lez-Vasallo (2006), low and high feed levels were calculated to provide the herring with a maintenance, i.e., low ration, diet and an above maintenance, i.e., high ration, diet, respectively. The high ration was equivalent to 0.45% of body weight per day and the low ration was equivalent to 0.11% of body weight per day. Both the high and low rations were fed to the fish four times per week. The total ration for each group was calculated by estimating the number of fish in each group (counted using an overhead photograph) and the average weight of the fish at the beginning of the experiment. Forty fish were sampled every month (except in November) in the same manner as in experiment 1 except that measurements with the fat meter were taken during every sampling event. Estimation of fecundity Fecundity was estimated by the auto-diametric method, using the principle that the number of oocytes per gram of ovary is a function of the average size of the oocytes (Thorsen and Kjesbu 2001). For ovaries containing vitellogenic oocytes, the diameters of 200 vitellogenic oocytes were measured using computer-aided automatic particle analysis. For ovaries that did not contain vitellogenic oocytes (i.e., contained only corticol alveoli or previtellogenic oocytes), 50 randomly selected oocytes were measured manually using a dissecting microscope and image analysis software. A principle requirement for accurate fecundity estimates using the auto-diametric method is that a representative sample of oocytes is measured. Atretic oocytes are Published by NRC Research Press

Kennedy et al.

19 Table 1. The dates when the herring were fed and the corresponding food ration given to each group (G) in (a) experiment 1 and (b) experiment 2. (a) Experiment 1. G1 G2 G3 G4

15 June–14 July 2 7 7 7

15 July–14 Aug. 7 2 7 7

15 Aug.–14 Sept. 7 7 2 7

15 Sept.–14 Oct. 7 7 7 2

(b) Experiment 2. G1 G2 G3

10 July–14 Aug. High Low Low

14 Aug.–18 Sept. Low High Low

18 Sept.–18 Oct. Low Low High

Note: In (a), values for each date range represent the number of days per week.

smaller and have a greater transparency than normal oocytes, so there is greater likelihood that these would not be measured during the analysis, leading to inaccuracies in the measurement of average oocyte diameter. For this reason, samples with an intensity of atresia greater than 15% were excluded from fecundity analysis. It was established previously that herring oocytes smaller than 240 mm are previtellogenic, those between 250 and 375 mm are cortical alveoli, and those larger than 375 mm are (true) vitellogenic (Ma et al. 1998; Kurita et al. 2003). From this it was decided that fish with a leading cohort (LC) (average diameter of the largest 10% of oocytes) greater than 250 mm were considered to have commenced maturation. ´ skarsFecundity was calculated using the equation from O son et al. (2002) for fish with an average oocyte diameter greater than 500 mm: F ¼ 1:708 1010 ðOW0:936 Þ OD2:301 ðn ¼ 71; R2 ¼ 0:967Þ where F is fecundity, OW is ovary weight (in grams), and OD is the average oocyte diameter (in micrometres). For fish with an average oocyte diameter between 375 mm and 499 mm, the following equation from Kurita and Kjesbu ´ skarsson et al. (2009) was used, as the equation from O (2002) did not include fish with an oocyte diameter below 500 mm: LogðOPDÞ ¼ ððVvto =Vova Þ ð1=ro Þ ðð1 þ kÞ3 =ð8 kÞÞÞ þ 12:28 3 logðOODÞ F ¼ OPD OW where Vvto/Vova is the volume fraction of vitellogenic oocytes in the ovary, 1/ro (1/specific gravity of the ova) = 0.949, and (1 + k)3 / (8k) (the correction factor due to the oocytes being non-spherical) = 1.077. The volume fraction of vitellogenic oocytes in the ova was calculated using the equation Vvto =Vova ¼ 0:0017 OOD This equation is from a regression line running between 375, 0.50 and 500, 0.71. The justification for this line is that

Vvto/Vova at 250 mm will be 0; extrapolation of the data in fig. 4 of Kurita and Kjesbu (2009) gives a Vvto/Vova of 0.5 at 375 mm. The Vvto/Vova at 500 mm is stated as being 0.71 in Kurita and Kjesbu (2009). Atresia The intensity of atresia was assessed using the profile counting method. Histological sections of ovaries were screened under the microscope for a stages of atresia (Hunter and Macewicz 1985). These sections were separated by a minimum distance that equalled the diameter of the leading cohort of oocytes from that ovary so that the same oocytes were not counted twice. At least 150 oocytes were counted, and the intensity of atresia was calculated as the percentage of atretic oocytes of all oocytes counted (i.e., atretic 100/(normal + atretic)). As atretic oocytes are smaller than normal oocytes, counting of atretic oocytes using the profile method can lead to an underestimation of the intensity of atresia. The intensity was corrected using the equation below from Gonza´lez-Vasallo (2006), who examined the intensity of atresia in the same ovaries using the profile and the unbiased physical disector method (Andersen 2003). This equation does not take into account the effects of the size of atretic oocytes; however, as there is a high correlation in the relationship, the effect of differences in the size of the atretic oocytes is probably small. RIA ¼ 3:0881 A0:75

ðn ¼ 19; R2 ¼ 0:93Þ

where RIA is the relative intensity of atresia and A is the intensity of atresia from profile counting. A fish was considered to be skipping if 15% of the oocytes present in the ovary were atretic or it had no oocytes in cortical alveoli or vitellogenic stage. Fifteen percent was regarded as an indication that fish would go on to reabsorb the remainder of their oocytes as below 15% appears to be the baseline amount for the ‘‘normal’’ gradual down-regulation of oocytes (Kurita et al. 2003). Calculations and statistics All statistics were carried out using Statistica 7.0 (Statsoft Inc., Tulsa, Oklahoma). Fulton’s condition factor (K) (see Nash et al. 2006) was calculated using the equation Published by NRC Research Press

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K¼

TW TL3

where TW is total weight (in grams) and TL is total length (in centimetres). Gonadosomatic index (GSI) was calculated using the equation GSI ¼

GW 100 TW

where GW is gonad weight (in grams) and TW is total weight (in grams).

Results Fish length, age, and condition In experiment 1, the fish were composed almost entirely (98%) of the 2002 year class of NSS herring, which were 4 years old at the start of the experiment, and so the majority would have been potential second-time spawners at the end of the experiment (the remaining 2% were from the 2003 year class). The fish had an average length of 28 cm at the start of the experiment, which increased to 30 cm by the end. In experiment 2, the fish had an average age of 7 years (standard deviation = 2 years) at the start of the experiment, with a small number of fish of ages 10+. The fish had an average length of 33 cm at the beginning and end of the experiment. In experiment 2, the fish were composed almost entirely of NSS herring; there was also a small number of autumn-spawning herring, which were easily identifiable as they exhibited a maturation pattern distinct from that of the NSS herring. Autumn spawners were excluded from all analysis. In both experiments, the fish were caught at the spawning grounds and therefore would have spawned at least once in their life; thus all fish in the experiments were repeat spawners. Fulton’s condition factor (K) was independent of length in all months in both experiments (linear regression; p > 0.05; the development of K can be seen in Figs. 2 and 3). In August in experiment 1, group 4 had a significantly lower K than the other three groups (analysis of variance, ANOVA; p 0.05). In October, this group had the highest K but there were no significant differences between any of the groups (ANOVA; p > 0.05). In December and March, there were no significant differences in K between any of the groups. In experiment 2, there were no significant differences in K between any groups in any month except for group 3, which had a significantly higher K in August that then returned to a level similar to those of the other groups in September. At the last sampling date, 66% and 85% of the fish in experiments 1 and 2, respectively, had K levels below 0.70; herring with a K less than 0.7 is considered to be of low condition as atresia shows a sharp rise in ´ skarsson et al. 2002). fish below this level (O GSI, fecundity, and oocyte diameter There were no significant differences in GSI, oocyte diameter (ANOVA; p > 0.05) (Figs. 2 and 3), or fecundity (ANCOVA, weight as covariate; p > 0.05) between groups in any month in either experiment. In experiment 1 (four

tanks combined), LC oocyte diameter was positively correlated with K in August (linear regression; n = 80, R2 = 0.28, p < 0.001) and March (linear regression; n = 60, R2 = 0.13, p = 0.002) (Fig. 4). In experiment 2 (three tanks combined), there was a positive linear relationship between LC oocyte diameter and K in September (n = 57, R2 = 0.22, P < 0.001), October (n = 49, R2 = 0.75, P < 0.001), November (n = 60, R2 = 0.62, P < 0.001), January (n = 62, R2 = 0.34, P < 0.001), and February (n = 53, R2 = 0.46, P < 0.001) (Fig. 4). There was no significant difference in the fecundity– weight relationships in February between groups or experiments (analysis of covariance, ANCOVA; n = 79, P > 0.05), so all data on fecundity was combined. A stepwise linear regression was carried out to find the best predictor of fecundity in February using length, weight, K, and fat content as variables. A combination of weight and LC oocyte diameter gave the best predictor of fecundity (linear regression; n = 79, R2 = 0.68, P < 0.001). Fecundity decreased from October through to March in experiment 1 (ANCOVA, weight as covariate; n = 165, P < 0.05) (Fig. 5) and experiment 2 (ANCOVA, weight as covariate; n = 108, P < 0.05) (Fig. 6), with a decrease of 20% and 44%, respectively (calculated from the average of all groups combined) between October and the end of the experiment. The average fecundity in experiment 2 showed an increase from September to October, indicating the fish are recruiting oocytes over this time. The explanatory power of weight and length on fecundity was assessed in different stages of ovary development (measured as LC oocyte diameter) using linear regression (Table 2). The fish from all months and both experiments were grouped according to LC oocyte diameter. Weight was the best predictor of fecundity over length for the majority of the groups. The addition of K into the fecundity–length regression explained additional variance for all the groups. When K was added to the fecundity–weight regression, it only explained additional variance for four of the eight groups and three of these were mainly for the lower size of oocyte diameters. Fat content (only available for fish in later stages of oocyte development) did not add any additional explanatory power. Atresia Ninety-one percent of the fish sampled in experiment 1 in March had atretic oocytes (Fig. 7). The average intensity of atresia was 9%, with 90% of the fish having an intensity of less than 15%. Atresia was less prevalent but more intense in the February samples of experiment 2, with atretic oocytes being present in 78% of the fish with an average intensity of 29% (Fig. 7). Several fish in experiment 2 had an atretic intensity of 100% and all of these had K < 0.7. Skipping spawning In experiment 1, 10% of the fish were considered to be skipping the coming spawning season, all of which were skipping by reabsorbing their oocytes through atresia (i.e., had atresia greater than 15%). The premise of skipping is based on the assumption that all of the fish had spawned in the previous year; otherwise they would simply be classed as ‘‘failing’’ to spawn. In experiment 2, a total of 60% of Published by NRC Research Press

Kennedy et al.

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Fig. 2. The change in (a) weight, (b) Fulton’s condition factor (K), (c) gonadosomatic index (GSI), and (d) leading cohort (LC) oocyte diameter with month in experiment 1: group 1, ^; group 2, &; group 3, ~; and group 4, !. Error bars = 1 standard deviation.

the fish were considered to be skipping. However, this was composed of 24% that had not begun ovary maturation and 36% that had atresia greater than 15%.

Discussion The focus of the present study was to subject herring to temporally variable feeding regimes to observe (i) how this affected fecundity and atresia and (ii) the incidence of skipped spawning. To do this we attempted to generate differences in condition (K) in different groups of herring at different times. In experiment 1, only one group had a lower K than the others at any sampling point. By the end of the feeding period, when all of the fish had received a similar amount of food, K was equal across all of the groups, which was also an aim of the experiment. Unfortunately, the fish were not sampled at all of the dates when the food ration was changed (this was rectified in experiment 2). In experiment 2, there were no significant differences in K between groups except for in one month. The reason for the lack of differences in K between groups may be that herring have more efficient digestion at lower food levels. Even so, the data for individual fish revealed interesting patterns between energy reserves, the onset of maturation, fecundity, and skipped spawning for herring. There was a difference in the type of food used between experiments. An aim of the study was for the fish in the two

experiments to differ in their amount of stored energy at the end of the feeding period. This was believed to be mostly influenced by the amount of food eaten and the calorific value of the food. Thus, it was considered that having the energy come from two different types of food would not influence the result. Support for this was given by there being no significant difference in the weight–fecundity relationship between the two experiments. ´ skarsson et al. (2002) considered that herring with K < O 0.7 was in poor condition due to a sharp rise in atresia in fish below this level of K. From the results, it appears that this level of K represents an important energy threshold for NSS herring as the fish in the experiment appeared to need to reach a K of about 0.70 to begin ovary maturation. Herring with a K above this value seem to have a 100% chance of beginning ovary maturation, whereas fish with a K < 0.70 are much less likely to begin ovary maturation. As herring only feed during the summer, they thus must achieve this level before the feeding season is finished. The timing of food acquisition had no effect on maturation, with condition having the strongest influence. This suggests that there is no critical period during the feeding season that affects the decision to mature. This is also supported by the fact that some of the herring began ovary development, even if they were below the threshold of 0.70. However, as LC diameter was correlated to condition, herring in poor condition started maturation later than better conditioned fish. It was also Published by NRC Research Press

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Fig. 3. The change in (a) weight, (b) Fulton’s condition factor (K), (c) gonadosomatic index (GSI), and (d) leading cohort (LC) oocyte diameter with month in experiment 2: group 1, ^; group 2, &; and group 3, ~. Error bars = 1 standard deviation.

seen that many of the fish did not begin ovary maturation until after October and some of these individuals were able to grow their oocytes quickly and ‘‘catch up’’ with the earlier maturing fish. Gonza´lez-Vasallo (2006) also found that herring fed on lower rations began ovary development much later in the year than those fed on higher rations and had a faster oocyte growth. How this higher growth will affect the quality of the eggs is unknown, but as the parental fish has low energy reserves, this might be reflected in a lower egg size or dry mass (Ouellet et al. 2001). Many of the low energy fish that began maturation had insufficient energy to complete the process and thus subsequently reabsorbed many of their oocytes by atresia. This suggests that the decision to begin the maturation process is influenced by more than just energy levels, probably day length (photoperiod), in particular (Bromage et al. 2001), and there may be communication within the shoal to synchronize ovary development and spawning. It has been indicated that weight many months before spawning is important for determination of fecundity in Atlantic cod (Skjæraasen et al. 2006) and plaice (Kennedy et al. 2007). This does not seem to be the case for herring as the timing of food acquisition appeared to make no difference to the final fecundity. The opposite actually appears to be true for herring (i.e., weight close to spawning is more important), as the predictive power of weight for fecundity

was very low at the beginning of ovary development and improved as ovary development proceeded. Thus, the downregulation of fecundity seems to play a greater overall role in the determination of realised fecundity (actual number of eggs spawned) in herring compared with cod. Both are capital spawners, but herring is an extreme example, i.e., most of its gonad development is funded from storage reserves (Slotte (1999) and references therein). They also begin ovary development between May and August (Kurita et al. 2003) but do not cease feeding until September (Slotte 1999). To achieve the maximum fecundity possible for their available energy reserves, it is probably best to recruit more oocytes than can be taken to full maturation and then decrease the number to a level of investment that matches the total energy reserves available at the end of the feeding season. This strategy has been documented both in wild NSS herring (Kurita et al. 2003) and in Pacific herring (Hay and Brett 1988). This could also allow herring to take advantage of any unexpected increases in food abundance and use this to fund the growth of the excess oocytes. This allows them to have a higher fecundity than would have been possible if they had based their fecundity on energy levels at the time of oocyte recruitment. Fecundity was gradually down-regulated with the decrease in weight and became more closely related to weight as ovary development proceeded, as can be seen by the Published by NRC Research Press

Kennedy et al.

23

Fig. 4. Leading cohort (LC) oocyte diameter versus condition for experiments 1 (&) and 2 (^) combined for (a) August, (b) October, (c ) December, and (d) February (experiment 1) and March (experiment 2). In (d), fish that had an atretic intensity greater than 15% are shown for experiment 1 (+) and experiment 2 (). The broken line parallel to the x axis indicates an oocyte diameter of 250 mm; herring with a LC oocyte diameter above this limit have began oocyte development. The broken line parallel to the y axis indicates a K of 0.7, an important energy threshold for herring.

Fig. 5. Average fecundity of the herring in experiment 1 (all four groups combined) at time of sampling in each month. Error bars = 1 standard deviation.

Fig. 6. Average fecundity of the herring in experiment 2 (all three groups combined) at time of sampling in each month. Error bars = 1 standard deviation.


24


Table 2. The number of samples (n) and R2 values for the fecundity–weight (W–R2) and fecundity–length (L–R2) relationships for herring at different intervals of leading cohort oocyte diameter. Oocyte diameter (mm) 350–450 450–550 550–650 650–750 750–850 850–950 950–1050 1050+

n 67 62 39 38 23 20 13 14

W– R2 0.32 0.43 0.58 0.77 0.73 0.83 0.84 0.79

KW –R2 0.71 0.58 0.63

0.90

L– R2 0.63 0.38 0.28 0.21 0.45 0.84 0.58 0.70

KL– R2 0.64 0.49 0.56 0.70 0.70 0.90 0.72 0.81

Note: KW–R2 and KL–R2 indicates the R2 of the fecundity–weight and fecundity–length regressions when Fulton’s condition (K) is added to the relationship; no value indicates that the addition of K did not explain any additional variance.

Fig. 7. Intensity of atresia versus Fulton’s condition factor (K) for fish sampled on the last sampling date for experiments 1 (&) and 2 (^).

change in explanatory power of weight for fecundity. Condition explained additional variance in the fecundity–weight relationship; this generally only occurs in the minority of fecundity–weight relationships (Koops et al. 2004) but, logically, would depend very much on the observed range in condition among examined individuals. The large spread in K in our experiment may have allowed us to pick up the effect of condition, as such a large variation in condition is generally not seen in wild populations of NSS herring in a single year (J. Kennedy, R.D.M. Nash, A. Slotte, and O.S. Kjesbu, unpublished data). Also, the increase in the predictive power of the relationship with the addition of K occurred mainly at the smaller oocyte sizes and the majority of fecundity studies estimate fecundity close to the end of the oocyte development, so this may be an important component only during early oocyte development.

The degree of down-regulation was different between the two experiments, with fish in experiment 2 showing a much greater level of down-regulation. A decrease in fecundity documented in the field between July 1998 and March 1999 showed a down-regulation of 56% (Kurita et al. 2003). The extent of down-regulation is probably related to the temporal pattern of change in K and the final K, with fish in experiment 2 ending up in much poorer condition than those in experiment 1. It has been seen in plaice that individual atretic loss over a specific period is correlated with the change in condition over the same time period (Kennedy et al. 2008), and captive Atlantic cod deprived of food during the spawning season spawned between 20% and 80% fewer eggs, depending on the nutritional status of fed individuals (Kjesbu et al. 1991). The degree of down-regulation is also probably related to oocyte diameters at the time of sampling as atresia is known to occur in fish with oocyte diameters up to 1200 mm (Kurita et al. 2003), thus the down-regulation would probably have been greater in both experiments if the experiments had been allowed to run longer, allowing the fish to progress further in oocyte development. This may also explain why down-regulation in the present study was lower than that in the study by Kurita et al. (2003) in which fecundity was estimated for fish that had more developed ovaries than in the present study. A previous study carried out by Ma et al. (1998) in which herring were kept for 1.5 years and fed on a high and low ration showed that the high-ration fish had a higher potential fecundity and also a lower level of atresia. Although the differences in potential fecundity between the two groups appeared to be mainly due to weight differences, the lowration group did have a much higher level of atresia. Gonza´lez-Vasallo (2006) also found that the differences in fecundity between groups of herring fed different amounts of food were mainly due to differences in weight. Studies carried out on other species have also found similar results (Yoneda and Wright 2005; Kennedy et al. 2008). The results from our study confirm the importance of weight (provided that the fish are in reasonable condition) as our results showed that the period in which food is consumed is not important for the determination of fecundity but that it is mainly determined by weight close to spawning. There were many fish in the present study that would not have been able to spawn at the end of the experiment and thus would be classed as skipping spawning. This skipping took on two forms: (ii) failure to begin ovary maturation, which is termed a resting spawner, and (ii) the complete reabsorption of the gonads, which is termed a re-absorbing spawner (Rideout et al. 2005). It appeared that a minimum K of 0.70 is required by herring to guarantee ovary maturation; below this value, there was a much lower chance of ovary maturation beginning. This skipping of reproduction without beginning ovary maturation has been witnessed in winter flounder (Burton 1994), turbot (Bromley et al. 2000), and Atlantic cod in the field (J.E. Skjæraasen, R.D.M. Nash, J. Kennedy, A. Thorsen, T. Nilsen, and O.S. Kjesbu, unpublished data) and in the laboratory (Skjæraasen et al. 2009) and is associated with low feeding and (or) low condition before the recruitment of vitellogenic oocytes. The majority of the fish with high atresia and all with an atretic intensity of 100% had K < 0.70. Fish with 100% atresia have previPublished by NRC Research Press

Kennedy et al.

ously been witnessed in the field, and they all had K of less ´ skarsson et al. 2002). In summary, K of 0.7 looks than 0.7 (O to be a very important threshold level for successful herring maturation. There were many fish with K > 0.70 that had atretic oocytes in the experiment, even though this is not witnessed ´ skarsson et al. in the field during the spawning season (O 2002; Kurita et al. 2003; J. Kennedy, personal observation). This is probably due to the development stage of the ovaries of the herring. According to the mentioned studies, atresia at low intensities is very prevalent in herring caught in October and November, when oocyte diameters are between 600 mm and 1200 mm. This is the period when the majority of the fecundity down-regulation takes place in the field. The fish in the experiment were slightly delayed compared with their natural cycle (probably due to the stress of capture), so many of the herring in February had oocytes in this range and thus many had atretic oocytes and much of the downregulation occurred later than November. It is difficult to know if our classification of fish into those that would skip spawning and those that would spawn successfully was correct. We considered a level of atresia that was above a ‘‘normal’’ intensity seen in the field (Kurita et al. 2003) to be an indicator of skipping spawning. However, it is difficult to know if the fish that were considered to have high atresia would actually have skipped. The complete re-absorption of all the oocytes is likely to be a gradual process rather than a single event of 100% atresia. ´ skarsson et al. (2002) predicted the final fecundity of fish O that exhibited atresia. They made several predictions based on a number of assumptions: whether oocyte development is linear or exponential and whether there is a broad or restricted size range of oocytes in which atresia can occur. From the present study and from Kurita et al. (2003), we ´ skarscan see that oocyte development is linear, and from O son et al. (2002), we can see that there is a broad range in ´ skarsson which atresia can occur. From these assumptions, O et al. (2002) found that an atretic intensity of 5% can lead to complete re-absorption of oocytes. However, this relies on the assumption that the atretic intensity remains constant throughout the entire period when oocytes are in the range where atresia can take place. This assumption is unlikely to be true, as atretic intensities have been seen to vary over time in individual fish (Kennedy et al. 2008). Engelhard and Heino (2005) suggested that up to 50% of second-time spawners skip their second spawning event. In experiment 1, 10% of the fish were considered to be skipping spawning. Almost all of the fish in experiment 1 would have been second-time spawners at the end of the experiment and thus would be comprised of the age class expected to be the most frequent skippers by Engelhard and Heino (2005). When the condition of these fish was compared with that of the wild stocks in terms of the percentage of fish with K < 0.70, i.e., the value considered to indicate that ´ skarsson et al. 2002), the a herring is in poor condition (O percentage in experiment 1 was shown to be higher than in ´ skarsson et the wild in any year between 1980 and 2000 (O al. 2002). In experiment 2, a significant proportion skipped, but the fish in this experiment were in extremely low condition and fish in such condition are seldom found in any significant numbers in the wild (Ndjaula 2009). The skipping

25

of reproduction appears to be closely linked to condition, so our results do not support the results of the study by Engelhard and Heino (2006). There are several reasons for fish to skip spawning: adverse environmental conditions, e.g., temperature, pH, etc. (which are not discussed here) during ovary maturation and spawning time, or energetic causes, which are not mutually exclusive from environmental causes. In relation to energetic causes, there are two main hypotheses as to why fish may skip: (i) to invest the energy that would have been used in reproduction for growth to attain a bigger size and hence a greater fecundity in later years (Holmgren 2003; Jørgensen et al. 2006), or (ii) they have too little energy and by spawning they risk death (Trippel and Harvey 1989; Jørgensen et al. 2006). It appeared that the skipping of reproduction in herring was mainly due to reason ii, i.e., non-spawning herring had insufficient energy reserves. It does appear that herring have a very optimistic view of the future, with herring in very low condition beginning ovary maturation even though they do not have the energy to support it; this is probably related to the long development time of the ovaries, about 8 months, and the patchy distribution and interyear variability in biomass of their food supply (Olsen et al. 2007).

Acknowledgements This study was funded by the Norwegian Research Council project 173341/S40 ‘‘Timing and determination of fecundity and skipped spawning: implications for stock– recruitment theory of determinate spawners’’. The authors thank the technical staff at the Institute of Marine Research, Matre Research Station, for maintaining the aquaria, feeding the fish, and assisting during sampling of the fish. The authors also thank the technicians at the Institute of Marine Research, Bergen, who read the scales, Bente Njøs Strand for assistance during analysis of gonad samples, and Sonnich Meier for help in planning the logistics of the second experiment.

References Andersen, T.E. 2003. Unbiased stereological estimation of cell numbers and volume fractions: the disector and the principles of points counting. In Modern approaches to assess maturity and fecundity of warm- and cold-water fish and squids. Edited by O.S. Kjesbu, J.R. Hunter, and P.R. Witthames. Institute of Marine Research, Norway. Fisken og Havet, 12: 11–19. Bagenal, T.B. 1966. Ecological and geographical aspects of fecundity of plaice. J. Mar. Biol. Assoc. U.K. 46(1): 161–186. doi:10.1017/S0025315400017628. Bagenal, T.B. 1969. Relationship between food supply and fecundity in brown trout Salmo trutta L. J. Fish Biol. 1(2): 167–182. doi:10.1111/j.1095-8649.1969.tb03850.x. Bromage, N., Porter, M., and Randall, C. 2001. The environmental regulation of maturation in farmed finfish with special reference to the role of photoperiod and melatonin. Aquaculture, 197(1– 4): 63–98. doi:10.1016/S0044-8486(01)00583-X. Bromley, P.J., Ravier, C., and Witthames, P.R. 2000. The influence of feeding regime on sexual maturation, fecundity and atresia in first-time spawning turbot. J. Fish Biol. 56(2): 264–278. doi:10. 1111/j.1095-8649.2000.tb02105.x. Burton, M.P.M. 1994. A critical period for nutritional control of Published by NRC Research Press

26 early gametogenesis in female winter flounder, Pleuronectes americanus (Pisces, Teleostei). J. Zool. (Lond.), 233(3): 405– 415. doi:10.1111/j.1469-7998.1994.tb05273.x. Corten, A. 2002. The role of ‘‘conservatism’’ in herring migrations. Rev. Fish Biol. Fish. 11(4): 339–361. doi:10.1023/ A:1021347630813. Dalpadado, P., Ellertsen, B., Melle, W., and Dommasnes, A. 2000. Food and feeding conditions of Norwegian spring-spawning herring (Clupea harengus) through its feeding migrations. ICES J. Mar. Sci. 57(4): 843–857. doi:10.1006/jmsc.2000.0573. Engelhard, G.H., and Heino, M. 2005. Scale analysis suggests frequent skipping of the second reproductive season in Atlantic herring. Biol. Lett. 1(2): 172–175. doi:10.1098/rsbl.2004.0290. PMID:17148158. Engelhard, G.H., and Heino, M. 2006. Climate change and condition of herring (Clupea harengus) explain long-term trends in extent of skipped reproduction. Oecologia (Berl.), 149(4): 593– 603. doi:10.1007/s00442-006-0483-3. Gonza´lez-Vasallo, B.D. 2006. Regulatory reproductive mechanisms and somatic growth in Atlantic herring (Clupea harengus L.). M.Sc. thesis, University of Bergen, Bergen, Norway. Gundersen, A.C., Nedreaas, K.H., Kjesbu, O.S., and Albert, O.T. 2000. Fecundity and recruitment variability of Northeast Arctic Greenland halibut during 1980–1998, with emphasis on 1996– 1998. J. Sea Res. 44(1–2): 45–54. doi:10.1016/S1385-1101(00) 00038-1. Hay, D.E., and Brett, J.R. 1988. Maturation and fecundity of Pacific herring (Clupea harengus pallasi): an experimental study with comparisons to natural populations. Can. J. Fish. Aquat. Sci. 45(3): 399–406. doi:10.1139/f88-048. Holmgren, K. 2003. Omitted spawning in compensatory-growing perch. J. Fish Biol. 62(4): 918–927. doi:10.1046/j.1095-8649. 2003.00086.x. Holst, J.C., Salvanes, A.G.V., and Johanesen, T. 1997. Feeding, Ichthyophonus sp. infection, distribution and growth history of Norwegian spring-spawning herring in summer. J. Fish Biol. 50: 652–664. doi:10.1111/j.1095-8649.1997.tb01956.x. Hunter, J.R., and Macewicz, B.J. 1985. Rates of atresia in the ovary of captive and wild northern anchovy, Engraulis mordax. Fish. Bull. (Washington, D.C.), 83: 119–136. International Council for the Exploration of the Sea. 2007. Report of the Working Group on Northern and Pelagic Blue Whiting Fisheries (WGNPBW). ICES CM 2007/ACFM:29. Jakobsson, J. 1969. On herring migrations in relation to changes in sea temperature. Jo¨kull, 19: 134–145. [Marine Research Institute, Reykjavik, Iceland.] Jørgensen, C., Ernande, B., Fiksen, Ø., and Dieckmann, U. 2006. The logic of skipped spawning in fish. Can. J. Fish. Aquat. Sci. 63(1): 200–211. doi:10.1139/f05-210. Kennedy, J., Witthames, P.R., and Nash, R.D.M. 2007. The concept of fecundity regulation in plaice (Pleuronectes platessa) tested on three Irish Sea spawning populations. Can. J. Fish. Aquat. Sci. 64(4): 587–601. doi:10.1139/F07-034. Kennedy, J., Witthames, P.R., Nash, R.D.M., and Fox, C.J. 2008. Is fecundity in plaice (Pleuronectes platessa L.) down-regulated in response to reduced food intake during autumn? J. Fish Biol. 72: 78–92. doi:10.1111/j.1095-8649.2007.01651.x. Kjesbu, O.S., Klungsøyr, J., Kryvi, H., Witthames, P.R., and Greer Walker, M. 1991. Fecundity, atresia, and egg size of captive Atlantic cod (Gadus morhua) in relation to proximate body composition. Can. J. Fish. Aquat. Sci. 48(12): 2333–2343. doi:10.1139/f91-274. Kjesbu, O.S., Witthames, P.R., Solemdal, P., and Greer Walker, M.

Can. J. Fish. Aquat. Sci. Vol. 67, 2010 1998. Temporal variations in the fecundity of Arcto-Norwegian cod (Gadus morhua) in response to natural changes in food and temperature. J. Sea Res. 40(3–4): 303–321. doi:10.1016/S13851101(98)00029-X. Koops, M.A., Hutchings, J.A., and McIntyre, T.M. 2004. Testing hypotheses about fecundity, body size and maternal condition in fishes. Fish Fish. 5: 120–130. doi:10.1111/j.1467-2979.2004. 00149.x. Kurita, Y., and Kjesbu, O.S. 2009. Fecundity estimation by oocyte packing density formulae in determinate and indeterminate spawners: theoretical considerations and applications. J. Sea Res. 61(3): 188–196. doi:10.1016/j.seares.2008.10.010. Kurita, Y., Meier, S., and Kjesbu, O.S. 2003. Oocyte growth and fecundity regulation by atresia of Atlantic herring (Clupea harengus) in relation to body condition throughout the maturation cycle. J. Sea Res. 49(3): 203–219. doi:10.1016/S1385-1101(03) 00004-2. Ma, Y., Kjesbu, O.S., and Jørgensen, T. 1998. Effects of ration on the maturation and fecundity in captive Atlantic herring (Clupea harengus). Can. J. Fish. Aquat. Sci. 55(4): 900–908. doi:10. 1139/cjfas-55-4-900. Morgan, M.J., and Rideout, R.M. 2008. The impact of intrapopulation variability in reproductive traits on population reproductive potential of Grand Bank American plaice (Hippoglossoides platessoides) and yellowtail flounder (Limanda ferruginea). J. Sea Res. 59(3): 186–197. doi:10.1016/j.seares.2007.12.001. Nash, R.D.M., Valencia, A.H., and Geffen, A.J. 2006. The origin of Fulton’s Condition Factor — setting the record straight. Fisheries, 31: 236–238. Ndjaula, H.O.N. 2009. Reproductive traits and recruitment variability of pelagic fish resources: combining experimental approaches and time series analyses. Ph.D. thesis, University of Bergen, Bergen, Norway. Olsen, E.M., Melle, W., Kaartvedt, S., Holst, J.C., and Mork, K.A. 2007. Spatially structured interactions between a migratory pelagic predator, the Norwegian spring-spawning herring Clupea harengus L., and its zooplankton prey. J. Fish Biol. 70(3): 799– 815. doi:10.1111/j.1095-8649.2007.01342.x. ´ skarsson, G.J., and Taggart, C.T. 2006. Fecundity variation in O Icelandic summer-spawning herring and implications for reproductive potential. ICES J. Mar. Sci. 63(3): 493–503. doi:10. 1016/j.icesjms.2005.10.002. ´ skarsson, G.J., Kjesbu, O.S., and Slotte, A. 2002. Predictions of O realised fecundity and spawning time in Norwegian springspawning herring (Clupea harengus). J. Sea Res. 48(1): 59–79. doi:10.1016/S1385-1101(02)00135-1. Ouellet, P., Lambert, Y., and Berube, I. 2001. Cod egg characteristics and viability in relation to low temperature and maternal nutritional condition. ICES J. Mar. Sci. 58(3): 672–686. doi:10. 1006/jmsc.2001.1065. Prokopchuk, I., and Sentyabov, E. 2006. Diets of herring, mackerel, and blue whiting in the Norwegian Sea in relation to Calanus finmarchicus distribution and temperature conditions. ICES J. Mar. Sci. 63(1): 117–127. doi:10.1016/j.icesjms.2005.08.005. Rideout, R.M., Rose, G.A., and Burton, M.P.M. 2005. Skipped spawning in female iteroparous fishes. Fish Fish. 6: 50–72. doi:10.1111/j.1467-2679.2005.00174.x. Rijnsdorp, A.D. 1990. The mechanism of energy allocation over reproduction and somatic growth in female North Sea plaice, Pleuronectes platessa L. Neth. J. Sea Res. 25(1–2): 279–289. doi:10.1016/0077-7579(90)90027-E. Skjæraasen, J.E., Nilsen, T., and Kjesbu, O.S. 2006. Timing and determination of potential fecundity in Atlantic cod (Gadus morhua). Can. J. Fish. Aquat. Sci. 63(2): 310–320. doi:10.1139/f05-218. Published by NRC Research Press

Kennedy et al. Skjæraasen, J.E., Kennedy, J., Thorsen, A., Fonn, M., Strand, B.N., Mayer, I., and Kjesbu, O.S. 2009. Mechanisms regulating oocyte recruitment and skipped spawning in the Northeast Arctic cod (Gadus morhua). Can. J. Fish. Aquat. Sci. 66(9): 1582–1596. doi:10.1139/F09-102. Slotte, A. 1999. Differential utilization of energy during wintering and spawning migration in Norwegian spring-spawning herring. J. Fish Biol. 54(2): 338–355. doi:10.1111/j.1095-8649.1999. tb00834.x. Thorsen, A., and Kjesbu, O.S. 2001. A rapid method for estimation of oocyte size and potential fecundity in Atlantic cod using a computer-aided particle analysis system. J. Sea Res. 46(3–4): 295–308. doi:10.1016/S1385-1101(01)00090-9. Thorsen, A., Marshall, C.T., and Kjesbu, O.S. 2006. Comparison of various potential fecundity models for north-east Arctic cod, Gadus morhua L., using oocyte diameter as a standardizing factor. J. Fish Biol. 69(6): 1709–1730. doi:10.1111/j.1095-8649.2006. 01239.x.

27 Trippel, E.A., and Harvey, H.H. 1989. Missing opportunities to reproduce: an energy dependent or fecundity gaining strategy in white sucker (Catostomus commersoni)? Can. J. Zool. 67(9): 2180–2188. doi:10.1139/z89-308. Witthames, P.R., and Greer Walker, M. 1995. Determinacy of fecundity and oocyte atresia in sole (Solea solea) from the Channel, the North Sea and the Irish Sea. Aquat. Living Resour. 8(1): 91–109. doi:10.1051/alr:1995007. Witthames, P.R., Thorsen, A., Greenwood, L.N., Saborido-Ray, F., Dominguen, R., Murua, H., Korta, M., and Kjesbu, O.S. 2009. Advances in methods for determining fecundity: application of the new methods to some marine fishes. Fish. Bull. (Washington, D.C.), 107: 148–164. Yoneda, M., and Wright, P.J. 2005. Effects of varying temperature and food availability on growth and reproduction in first-time spawning female Atlantic cod. J. Fish Biol. 67(5): 1225–1241. doi:10.1111/j.1095-8649.2005.00819.x.
