Russian Journal of Marine Biology, Vol. 29, No. 3, 2003, pp. 185–188. Original Russian Text Copyright © 2003 by Biologiya Morya, Zavolokin.
ECOLOGY
Size Correlation Between Predator and Prey in Some Fish Species of Nekton of the Sea of Okhotsk A.V. Zavolokin Far East State University, Vladivostok, 690600 Russia e-mail:
[email protected] Received January 13, 2003
Abstract—The coefficients of linear and exponential equations approximating the correlation between the length (mass) of a predator and the length (mass) of its prey were considered. The feeding habits of Alaskan pollack, chum salmon, and Pacific herring, considered as predators, were assessed by the value of the angular coefficient of respective regression equations. The bimodal distribution of feeding objects of herring was recorded. Key words: fish feeding, predator, prey, modeling, Alaskan pollack, chum salmon.
For the first time the correlation between the body length (mass) of predators and their prey was mathematically formulated by Ivlev [4]. He considered the correlation between sizes of food objects and of the predator feeding on it as a factor of selective feeding, for the data obtained in experiments. Further, a similar rule was established not only for fishes, but also for predator invertebrates [5]. Detailed analysis of the correlation between the dimensions of predators and their prey for bottom species of fishes was carried out by Scharf with colleagues [8], which had estimated the interspecific distinctions of the influence of the body size of a predator on the size of its trophic spectrum. The correlation between the size of predators and their prey was also discussed in series of particular works considering the feeding of Far East fishes [1, 3]. The present paper is devoted to research on the dependence between the length (mass) of a predator and the length (mass) of its prey. Alaskan pollack Theragra chalcogramma, Pacific herring Clupea pallasi, and chum salmon Oncorhynchus keta from the Sea of Okhotsk were considered as predators. The selection of objects for study was made because they were mass species with a wide range of body size variation that had not yet been studied in this respect. MATERIALS AND METHODS There were studied 98 individuals of Alaskan pollack, 17 individuals of herring, and 43 individuals of chum salmon sampled by trawls RT/TM 108/528 and RT 80/396 at 43 stations during a trawling survey of R/V “Professor Kaganovsky” in the Sea of Okhotsk in the months August–November in 2000. The salmon survey was conducted in the south part of the Sea of Okhotsk. The sampling sites of Alaskan pollack ranged
to the northeast, northwest, and central parts of the Sea of Okhotsk; Kamchatka; and the Sakhalin shelf. The herring was caught in the course of trawling mostly in the north regions of the sea. The trawling was conducted mostly in the upper layer, therefore all obtained results extended over epipelagic communities only. In each individual species of the fishes studied, the body length and mass were measured. The length of the fishes was measured from the beginning of the head up to the end of the middle rays of the tail fin. The length and mass of the species studied were respectively 5– 75 cm and 1.5–2810 g in the Alaskan Pollack, 14– 82.5 cm and 25–8425 g in chum salmon, and 15.5– 30 cm and 31–310 g in the Pacific herring. In the analysis of the stomach contents of fishes the weight average length of the body of each prey species was estimated, which further averaged the accounting to the relative number of species in the assay. The feeding objects were determined up to species or were sorted by their belonging to the following taxonomic groups: krill (Euphausiacea), hyperiids (Hyperiidae, Amphipoda), copepods (Copepoda), and arrowworms (Sagitta, Chaetognata). Fishes with poorly digested food were used to obtain more exact results. Copepods were measured from the rostrum up to the base of the caudal setae, The length of antennas was not taken into account in the measurement of the body length of krill and hyperiids. As the mass of the food components depended on their degree of digestion, a recovered (estimated) mass of the prey body was used to construct regressions. The mass of the body of copepods was determined by Chislenko nomograms [6]; that of other zooplankters, by the tables of Lubny-Hertsyk or of Mikulich and Rodionov [6]. The body mass of nekton representatives was calculated after regressions obtained from the
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results of measurements of the length and mass of the fish body in catches (herring and capelin) or by weighing their poor digested remains. The average mass of the prey was estimated also in the same way as the average length. Linear, and if necessary, exponential regression, equations of type y = ax, y = ax + b, and y = exp(b + ax) were estimated based on the obtained data, where x is the length (mass) of the fish body; y is the average length (mass) of the body of the prey; and a and b are the angular and linear coefficients of regression, respectively. Furthermore, 95% confidence boundaries of the a and b coefficients and the respective correlation and determination coefficients were assessed. RESULTS AND DISCUSSION Alaskan pollack, as the most common and the most relevant food fish of the Far Eastern seas, absolutely prevails in epipelagic communities, making up about 80% of the biomass of all epipelagic fishes in the Sea of Okhotsk [1]. This fact largely determined the leading role of the pollack in hydrobiont consumption. The food spectrum of the pollack depends on the number of potential feeding objects in the given area. Thus, its feeding conditions were determined not only by prey abundance, but also by the pattern of their spatial distribution [7]. The alimentary spectrum of the pollack included food objects with an average size of 0.19–28.5 cm and an average mass of 0.00046–175 g. Animals of eight taxonomic groups were found in the fish stomachs. The feeding base was laid by krill, which occurred in 2/3 of the examined stomachs. Pearson’s coefficient of correlation between the length (mass) of the pollack’s body and the length (mass) of the food objects was 0.65 (0.57). The linear regression of the dependence of the average length of food objects (y) on the length of the pollack (x) looked like y = (0.0896 ± 0.0185)x (figure). The correlation between the body mass of prey (y) and predator (x) was described by the equation y = (0.0129 ± 0.0039)x. The determination coefficient (R2) of regression was 0.26 for body length and 0.27 for body mass. Such low values were determined by the feeding specific of the pollack. Individuals with a length less than 40–45 cm feed mostly on plankton; with size increase the ratio of nekton prey increased, however, plankton still played a relevant part in nutrition. As result, variation of prey length reached 10 cm for the pollack of the 45–60 cm size group. Finally, the pollack with a length exceeding 60 cm feed mostly on large nekton, which was manifested by a sharp increase in the average length of its prey. Based on this, the investigated dependence could be described by a nonlinear function. In an exponential model by body length such a dependence looked like y = ( 0.154 ± 0.036 )e
( 0.056 ± 0.010 ) x
( R = 0.56 ) 2
by mass, y = ( 0.0096 ± 0.0009 )e
( 0.0034 ± 0.0004 ) x
( R = 0.71 ) . 2
(see figure). The angular coefficient of linear regression can be determined also by averaging the y/x ratio. Then the equation of linear regression by body length will become y = (0.0623 ± 0.0138)x, and by mass, y = (0.00274 ± 0.00176)x. The significance of chum salmon in communities of epipelagic fishes is not so great as that of pink salmon. However, it is worthwhile to note that among all salmons the chum is the most pronounced planktoneater [2]. The preference of this or that kind of food varies with augmentation of the dimensions of the chum. The food base of a small chum (10–30 cm) is comprised of copepods, hyperiids, and krill. The pteropod Clione limacine began to prevail in the diet of larger individuals. The share of nekton (fishes and squids) was insignificant and was characterized most often by occasional presence. The dietary spectrum of chum salmon included feeding objects of 0.15–15 cm length and of mass 0.0001–43.8 g. The correlation coefficients of body length and mass were significant and equal, respectively, to 0.74 and 0.89. The equation of linear regression by body length looked like y = (0.0351 ± 0.0080)x. The dependence of the mass of the prey on the mass of the predator was manifested by the equation y = (0.00151 ± 0.00025)x (figure). The determination coefficient in these instances was 0.46 and 0.77, respectively. The equation of regression, the angular coefficient of which was determined by the y/x ratio for the body length of the predator and its prey looked like y = (0.0264 ± 0.0048)x, for their body mass it was y = (0.000181 ± 0.000140)x. The Pacific herring is one of the most widespread and mass commercial species in the North Pacific. In 1997–1998 it took the second place by volume of catches in the Far Eastern seas after Alaskan pollack. The food spectrum of the herring includes objects of 0.15–2.05 cm length and of 0.0002–0.09 g mass. The angular coefficients did not significantly differ from zero for equations of the type y = ax and y = ax + b for body length and mass (figure). The correlation coefficients of the body length and mass were rather low, −0.29 and −0.44, respectively. This resulted from the fact that the food objects of herring were specified by two certain dimensional groups (manifested by two “clouds” of points). The ratio of intergroup and intragroup dispersion was calculated to assess their significance. The obtained value was compared to 1 with the Fisher criterion. The zero-hypothesis on the equality of dispersions was declined with 99.9% probability, which confirmed the significance of the distinguished clusters.
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SIZE CORRELATION BETWEEN PREDATOR AND PREY
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Theragra chalcogramma 35
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Mean prey mass, g
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0 100 Clupea pallasi
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Dependence between the size (to the left) and mass (to the right) of the body of predatory fishes and of their prey. Continuous lines— linear dependence, dotted line—exponential dependence.
This was laid in the base of the correlation assessment of each group distinguished. The correlation coefficients for the first and the second group were, respectively, 0.046 and –0.097 for length, and –0.29 and − 0.44 for mass. The zero-hypothesis on equality to zero of the length and mass angular coefficients for both groups was accepted with 95% probability, which disRUSSIAN JOURNAL OF MARINE BIOLOGY
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played the lack of a significant correlation between the body length (mass) of a predator and its prey. The values of the regression coefficients enable us to consider, first, the rates of the increase of the average size of prey with the growth of the predator, and, second, the degree of distinction of the dimensional parameters of food objects of the fish species studied. A No. 3
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comparison of the coefficients of linear regression showed that the average length of the prey of the Alaskan pollack increased 2.5 times, and the mass increased 8.5 times faster than those of the chum salmon. The zero-hypothesis of equality of the angular coefficients of Alaskan pollack and of chum salmon by body length and mass was declined by Student’s criterion with a 99% probability. In the instance when the regression was constructed for Alaskan pollack less than 60 cm, the angular coefficients for pollack and herring did not differ significantly with 95% confidence.
ACKNOWLEDGMENTS The author is indebted to Drs. A.J. Efimkin and N.A. Kuznetsova (TINRO- center) for useful recommendations for sampling and processing of the material. This work was supported by CRDF, the Ministry of Education of the Russian Federation and the program “Financial Support of Scientific Programs in DVO RAN in 2002.”
The lack of a size correlation between predator and prey in herring is possible to explain by the uniformity of its food objects. The biomass of zooplankton is mostly presented by crustaceans, the bodies of which have a similar structure and a small range of variation of size (compared to the length of thte predator). Therefore, the composition of nutrition depends on the morphology of fishes or on their size, and the size of food species does not differ significantly between juveniles and adult fishes.
1. Volkov, A.F., Gorbatenko, K.M., and Efimkin, A.Ya., Feeding Strategy of the Alaska Pollack, Izv. TINRO, 1990, vol. 111, pp. 123–132. 2. Volkov, A.F., Efimkin, A.Ya., and Chuchukalo, Regional Specifics of Feeding of Pacific Salmons in summer season, Izv. TINRO, 1997, vol. 122, pp. 326–327. 3. Gorbatenko, K.M., Feeding of Juveniles of Pink and Chum Salmons in Epipelagial of the Sea of Okhotsk in Winter Season, Izv. TINRO, 1996, vol. 119, pp. 234–243. 4. Ivlev, V.S., Eksperimental’naya ekologiya ryb (Experimental ecology of fishes), Moscow: Pischepromizdat, 1955. 5. Monakov, A.V., Pitanie presnovodnyh bespozvonochnyh (Feeding of Fresh-water Invertebrates), Moscow: RAN, 1998. 6. Rekomendatsii po ekspress-obrabotke setnogo planktona v more (Recommendations on Express–processing of Net Plankton in the Sea), Vladivostok: TINRO, 1984. 7. Shuntov, V.P., Volkov, A.F., Temnykh, O.S., and Dulepova, E.P., Mintai v ekosistemakh dal’nevostochnyh morei, (Alaska Pollack in Ecosystems of the Far East Seas), Vladivostok: TINRO, 1993. 8. Scharf, F.S., Juanes, F., and Rountree, R.A., Predator Size–Prey Size Relationship of Marine Fish Predators: Interspecific Variation and Effects of Ontogeny and Body Size on Trophic-niche Breadth, Mar. Ecol. Prog. Ser., 2000, vol. 208, pp. 229–248.
Thus, ontogenetic distinctions in the feeding of the fish species considered from the Sea of Okhotsk are manifested in the increase of angular coefficients of linear regression in the series “herring—chum—pollack”. This is apparently related to the specifics of the food preference of the fish species considered. The herring feeds mostly on plankton. There is some portion of nekton alongside plankton in the nutrition of the chum salmon. The Alaskan pollack is an everyphagous species; the plankton portion gradually decreases with growth in its nutrition, and upon reaching a certain size (about 60 sm) the pollack switches to feeding on nekton. As a result the dependence between the size of Alaskan Pollack and its food objects is more precisely described statistically by an exponential function.
REFERENCES
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