Marine Biology (2005) 148: 167–177 DOI 10.1007/s00227-005-0059-5
R ES E AR C H A RT I C L E
E. Macpherson Æ N. Raventos
Settlement patterns and post-settlement survival in two Mediterranean littoral fishes: influences of early-life traits and environmental variables
Received: 6 May 2004 / Accepted: 3 June 2005 / Published online: 20 July 2005 Springer-Verlag 2005
Abstract We examined the relationships between daily pattern of settlement and environmental parameters during two consecutive years in two littoral fishes, Lipophrys trigloides (Blenniidae) and Chromis chromis (Pomacentridae), in the NW Mediterranean Sea. We also used individual early-life traits (pelagic larval duration, size at hatching and size at settlement) calculated from otoliths, to study the proximate causes of settlement variability and size-selective mortality after settlement. Several early-life characteristics of L. trigloides (planktonic larval duration and size at hatching), and environmental variables averaged during the whole planktonic period (e.g. water temperature, wave height, solar radiation) were related with the magnitude of settlement. In contrast, C. chromis showed no significant relationships between early-life traits and the magnitude of settlement, and a weak relationship between settlement magnitude and environmental variables. Furthermore, juvenile survivors showed larger size at hatching than settlers, indicating that size at hatching affected the juvenile survival of the two species. These results suggest that survival was linked largely to conditions at hatching for both species.
Introduction The early-life traits of fishes may play a decisive role in settlement success, and small differences in larval characteristics may result in large fluctuations in the numbers of individuals that settle into juvenile habitats (Campana Communicated by S.A. Poulet, Roscoff E. Macpherson (&) Æ N. Raventos Centro de Estudios Avanzados de Blanes (CSIC), C. acc. Cala Sant Francesc 14, 17300 Blanes (Girona), Spain E-mail:
[email protected] Tel.: +34-972-336101 Fax: +34-972-337806
1996; Bergenius et al. 2002; Wilson and Meekan 2002; Takasuka et al. 2004). In addition, the early-life traits may condition individual survival in the months following settlement, and individuals that exhibit a larger size at hatching and/or faster larval growth may enjoy a greater likelihood of surviving (Searcy and Sponaugle 2000, 2001; Shima and Findlay 2002; Vigliola and Meekan 2002; Raventos and Macpherson 2005a). The relationships between early-life traits and settlement success and post-settlement survival are generally explained using the ‘‘growth–mortality hypothesis’’, which holds that small fish have a lower probability of surviving than larger fish at the same age (Anderson 1988; Bailey and Houde 1989). This well-known hypothesis postulates a series of mechanisms that explain its operation, namely, ‘‘bigger is better’’, which holds that juvenile mortality is negatively size dependent; ‘‘stage duration’’, which assumes that faster growing larvae are better able to survive the larval stage because they have a lower likelihood of being affected by mortality events; and ‘‘growth-selective predation’’, which assumes that growth rate directly impacts individuals’ vulnerability to predation (Meekan and Fortier 1996; Vigliola and Meekan 2002; Takasuka et al. 2003, 2004; Brown et al. 2004; Raventos and Macpherson 2005a). Certain early-life traits (e.g. size at hatching) may be influenced by attributes of the parent stock (Chambers et al. 1989; McCormick 2003; Green and McCormick 2005), whereas other traits (e.g. planktonic larval duration, growth rate, size at settlement) are heavily dependent on such environmental variables as water temperature and turbulence (Sponaugle et al. 2002; Meekan et al. 2003; Hutchinson and Hawkins 2004; Raventos and Macpherson 2005b), which means that the conditions borne by the larvae over the course of the time they spend in the plankton could well be conditioning factors affecting settlement success and postsettlement survival (Heath 1992; Wilson and Meekan 2001, 2002). We selected two common littoral fish species, Lipophrys trigloides (Blenniidae) and Chromis chromis
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(Pomacentridae) in the NW Mediterranean Sea to study the proximate causes of variability in the temporal patterns of settlement and size-selective mortality after settlement. Both species lay eggs on rocky areas adjacent to the shore but have distinct larval durations and settlement patterns. L. trigloides lives in the upper layer of the rocky shores, between 0 and 1 m (Macpherson 1994), and has a long and variable planktonic larval duration (PLD, 37–71 days; Raventos and Macpherson 2001). The number of settlers arriving each day is very small (20– 50 ind./shoal) only in the summer (Garcı´ a-Rubies and Macpherson 1995; Raventos and Macpherson 2001). During two settlement seasons (2001–2002 for L. trigloides, 2000–2001 for C. chromis) individual early-life traits (size at hatching, planktonic larval duration, and size at settlement), all based on otolith measurements, were used to test: (1) the effect of certain environmental variables averaged over the PLD on early-life traits; (2) the effect of both individual larval characteristics and environmental variables averaged over the PLD, on settlement success; and (3) the effect of variation in the early-life traits on post-settlement survival.
Materials and methods Field sampling Lipophrys trigloides Specimens were collected along the coast of Blanes, Spain (4140.9¢N, 247.9¢E, NW Mediterranean) (Fig. 1). Censuses of settlers were conducted daily from 1 February to 15 June 2001 and 2002, thereby covering the entire settlement period. Twenty transects of the same length (20·1 m) having the same habitat features (bare rock at 0–1 m in vertical walls) were identified within the study area by topo-
Fig. 1 Map of the study area, including the sampling zones for Lipophrys trigloides (black) and Chromis chromis (grey) and the stations where environmental variables were recorded, Lloret (filled triangle metereological data) and Tordera (filled star oceanographic data)
graphical features or by markers anchored to the rock. This habitat is dominant in the study area. During each census all settlers on three transects randomly selected from the list of transects were counted and collected by snorkelling with hand nets. Newly settled fish were nearly translucent and easily distinguished from postsettlers. All individuals collected at the three sampled transects on a given day were pooled together to yield overall daily totals. Individuals were measured in the laboratory and the otoliths (sagittae) were removed. When bad weather prevented censusing and collection, all settlers were collected on the next calm day, and the day of settlement was determined by otolith reading. All juveniles at three randomly selected replicate 200·1 m transects at 0–1 m were counted and collected 1 month after the arrival of the last settler (end of June). Juveniles were measured and the otoliths were removed in the laboratory. Chromis chromis Specimens were collected in the same area than in L. trigloides, between 5 and 16 m (Fig. 1). Censuses of settlers covered the entire settlement period and they were conducted daily from 1 July to 15 August 2000 and 2001. Daily censuses were carried out by SCUBA diving in three areas (approx. 100 m2 each) isolated from each other by wide, sandy expanses or by rocky walls and from other adjacent settlement areas. All settlers in each of the shoals (5–8 schools per area) were counted. All individuals collected at the three sampled areas on a given day were pooled together to yield overall daily totals. All shoals stayed in the same place early in the settlement season, although some schools aggregated in larger shoals 2–3 weeks later. No juveniles were ever observed in the zones between the three sampling areas, hence the settlement patterns observed at each sampling site were not affected by migrations from other areas. Settlers were quite readily distinguishable from older,
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established individuals on the basis of size, the shape of the caudal fin (always very short in new settlers), and position in the shoal (new settlers were always observed near the bottom, whereas older ones tended to be more scattered in the water column). A subsample of roughly equal number of settlers from each area (9–30 individuals) was collected every week to estimate the early-life traits (see below). These specimens were measured and their otoliths (lapilli) were removed in the laboratory. The density of juveniles 15 days after the arrival of the last settler (late August) was also estimated. All juveniles in the three areas were counted and a subsample was collected from each area. These specimens were measured and the otoliths removed in the laboratory. Otolith analyses For both species otoliths were used to estimate the size at hatching and age and length at settlement for two groups of individuals, namely, a ‘‘settler’’ group and a ‘‘juvenile’’ group. Otoliths were mounted on a glass slide and ground on lapping film to produce a thin transverse section. Age readings were effected and the presence of settlement marks was determined using a light microscope connected to a digital camera and an image analysis system (i.e. Wilson and McCormick 1999; Raventos and Macpherson 2001). Twice a single reader read all otoliths. Where increment counts differed by >5%, the otolith was discarded. In all, 60 L. trigloides otoliths (18.6% of the total read) and 55 C. chromis otoliths (13.8% of the total read) were discarded because of inconsistent readings. The data analysis employed 322 L. trigloides otoliths (sagittae) and 399 C. chromis otoliths (lapilli). Otoliths were read along the longest axis from the nucleus out to the edge. Otolith size at hatching (distance to the increment closest to the nucleus), PLD and otolith size at settlement were recorded. Length measurements were repeated three times to minimise measurements errors and the average values were used. The first increment was assumed to form on the day of hatching (Vigliola et al. 2000; Raventos and Macpherson 2005a). The ages of settlers and juvenile survivors were used to back-calculate the birthdate frequency distribution for each group. Daily growth could not be determined because many of the otoliths of both species exhibited an opacity along the longest axis that made it impossible to obtain an exact measure of each daily increment. This feature did not, however, prevent measurement of the other variables. To validate daily patterns of increment formation six settlers from each species were placed in a tank and acclimated for several days. They were next kept in a solution of 500 mg/l oxytetracycline in seawater for 24 h and returned to the tank for 10 days and then sacrificed. The number of increments following the fluorescent mark on the otolith was compared with the number of days after treatment. In all cases, increment counts were
10 and hence the interval of increment deposition was assumed to be daily. Environmental variables Weather data were obtained from an automatic weather station operated by the XMET service (National Weather Service) and situated 1 km from the sampling area (Fig. 1). Wind speed and component, solar radiation, and atmospheric pressure. Wind data were monitored every half hour with an anemometer mounted on a tower 10 m above the ground, and solar radiation and atmospheric pressure were also recorded at the same time intervals. Daily averages were calculated, and wind speed and direction were combined into a single coarse wind component. Winds blowing from E to SSW (90– 210) were classified as positive (onshore) and from 210.1 to 89.9 as negative (offshore; see Wilson and Meekan 2001; Raventos and Macpherson 2005b). Mean daily atmospheric pressure (hPa) and solar radiation (W m 2) values were also calculated. Significant wave height (defined as the mean of the highest third of the wave height readings recorded) values were obtained from a surface wave buoy (WANA Buoy no. 27053, http://adan.cedex.es/cgi-bin/toma_codigoNew.cgi?WN&27053) anchored at a distance of 15 km from the study area (Fig. 1). Data were recorded every 3 h and were averaged for each 24-h period (Raventos 2004). Sea surface temperature time series readings taken 5 m below the surface at a station 50-km distant consisted of twice-weekly readings recorded by CTD, which yielded values to an accuracy of two decimal places (Fig. 1). These data have been collected continuously since 1973. Before use, the time series readings were correlated with spot temperature readings (n=30) made in situ at the sampling sites over the settlement season, which displayed a high level of correlation (r>0.85, P
