WHY DO SOME PELAGIC FISHES HAVE WIDE FLUCTUATIONS IN THEIR NUMBERS? ---BIOLOGICAL BASIS OF FLUCTUATION FROM THE VIEWPOINT OF EVOLUTIONARY ECOLOGY--- by Tsuyoshi Kawasaki Faculty of Agriculture Tohoku University 1–1 Amamiya-cho Tsutsumi-dori Sendai-shi 980, Japan Resumen Los patrones de fluctuación en número de individuos varían de una especie (sub-población) a otra, los que han sido seleccionados a través del pro- ceso de evolución. En el caso de teleósteos marinos se presentan tres tipos extremos de patrones de fluctuación, IA, IB y II, representados respectiva- mente por saury y amodites, arenque y sardina, atunes y peces planos. Una relación entre estos tres tipos puede ser expresada por un triángulo con dimensiones de longevidad, fecundidad y tasa de crecimiento. El tipo IA, muestra cambios espaciados de breve tiempo, es una especie de vida corta, tiene una fecundidad baja, y el producto de k (parámetro de crecimiento de la ecuación de Bertalanffy) y T (tiempo generación) es bajo, lo que hace que la tasa instantánea de incremento natural de la población (r) sea alta, mientras que el tipo IB, se caracteriza por mostrar variaciones fenomenales de largo tiempo, son especies de vida larga, menos fecundas y el producto de kT es alto, y además acumulan una gran cantidad de peces cuando se presenta una sucesión de clases anuales fuertes a pesar de una r baja. El tipo II tiene una biomasa estable, son de vida larga, son más fecundos y el producto de kT es bajo así como r. El patrón de fluctuación específico para esta especie depende mucho de las condiciones ambientales bióticas y abióticas en que se desarrolla la especie. Las características de la ecología y el ambiente en que se desarrolla cada especie o su población tienen que ser tomadas en cuenta al momento de introducir medidas de ordenación de la pesquería. Se proponen medidas de ordenación para peces pelágicos y se hace una crítica de los modelos dependientes de la densidad en uso actualmente. Los peces que se alimentan de plancton están situados mucho más cerca a la fuente de energía solar, y por lo tanto parecen ser afectados fuertemente por los cambios del clima a través de los cambios oceánicos. Esta observación puede ser válida en particular para sardinas, las que son casi exclusivamente herbívoras. El problema es que mientras en algunos años utilizan el fitoplancton en forma eficiente, lo que da por resultado stocks muy grandes, en otros años no utilizan el fitoplancton tan eficientemente y se producen reducciones en su biomasa. Aparentemente el patrón de vida de las sardinas cambia. Si las condiciones ambientales son favorables, las sardinas aumentan en número adoptando un modo de vida que les permite mantener su población a un nivel máximo posible. Por el contrario, si las condiciones ambientales de la sardina son adversas y su nicho ecológico se contrae, estas se preparan para el siguiente 491 período de prosperidad asumiendo un estilo de vida en el cual se regula el crecimiento de la población. INTRODUCTION It has been well known that clupeoid fishes, especially the herrings and sardines, have phenomenal fluctuations in catch, viz. stock size. From of old, a variety of controversies have been held around the cause of such fluctuations and above all a dispute about the Californian sardine distributed off western North America is characteristic. Stock size of the Californian sardine drastically declined after a peak in 1936 (Fig. 1). The dispute around the collapse of the sardine stock has been continuing between scientists of California State standing on the conservation of the sardine stock and those of the Federal Government of the U.S. standing on the promotion of the fisheries (Radovich, 1981). In a paper jointly prepared by Clark and Marr (1955) each scientist reached an opinion different from the other based on the same data. Clark contended an influence of the exploitation on recruitments, saying that there had been a density-dependent effect between the stock and recruitment because if the stock size of the adult had been small, the resultant level of the progeny tended to become low. On the other hand, Marr asserted an effect of the environment on the recruitment, viz. density-independent effect, suggesting that the relation between the stock and recruitment was obscure. The same circumstances as above have been seen in the Far Eastern sardine, the catch of which dramatically decreased after a peak in 1937 (Fig. 1). Nakai (1962), a representative researcher on the sardine in Japan, tried to explain this event by the meandering of the Kuroshio. The stream axis of the Kuroshio, which flows eastward along the southern coast of Japan, often shifts, adding a semicircular southward movement. This in turn creates a counter clockwise cold eddy to the north and a clockwise warm eddy to the west. It is said that there are five types of pathway. One is normal or non-meandering (N), while the others meander in somewhat different patterns (types A- D in Fig.2). Although cold eddies associated with types B-D have occurred frequently, large-scale A-type fluctuations seldom have appeared. Nakai (1962) presumed that the spawning of the sardine was concentrated on a region south of Kyushu, Japan, and their eggs and newly hatched larvae drifting eastward came across an A-type cold eddy, resulting in a mass mortality. On the contrary, Cushing (1971) regarded a disastrous decline in recruitment of the sardine resulting from the heavy fishing as a cause of the collapse of the stock. While controversies between overfishing and natural cause around the decrease in stock of the sardines have been long held, similar disputes have also occurred about the herrings. 492 Fig. 1. Large-scale variations in catch of three species of sardine, Far Eastern, Californian and Chilean. Fig.2. Types of meandering of the Kuroshio path south of Japan. 493 CHANGE IN NICHE SIZE The number of organisms of a species varies from generation to generation. Let us consider the meaning of this variation. That a species continue to survive means that the maximal quantity of the matter and energy possible are taken over from a generation to the subsequent one. According to Simpson (1952), the number of species currently existing is 2 millions, while that which have been extinguished to date is one half billion. This implies that most species failed to achieve persistent replacement from generation by generation, eventually disappearing from the earth. This also means that a small number of species could remain surviving at present. The author considers that the success or failure of the persistence of organisms (viability) from one generation to the next is closely linked with the status and problems involved in the ecological-niche in the community concerned. “Niche” is defined as the status of a species in a community composed mainly of predator-prey relations. This is provided by the overall biotic and abiotic environment surrounding the species. To maximize the quantity of surviving biomass between generations means that a species regulates its biomass so that the size of its niche is filled completely by its organisms. To regulate the biomass means to regulate the number of organisms. If a population does not increase sufficiently to fill its niche once it has been extended, this niche would be “violated” by another population, the niche of which is close to the former. On the other hand, if a population does not decrease its biomass in response to the contraction of its niche, many organisms of that population would die or become weakened due to overpopulation. Only a species which had acquired the ability to regulate its number so that it fills the niche completely in response to fluctuations in niche size could have survived. The response pattern to fluctuations in niche size depends on species. Therefore, the pattern of fluctuation in number differs from species to species and this is called species-specific pattern of fluctuation in number. The species-specific pattern of fluctuation in number is the pattern of resource utilization for a species, which is having been formed through evolution and history. EXTREME TYPES AND DEVELOPMENTAL PROCESSES OF THE PATTERNS OF FLUCTUATION IN NUMBER Patterns of fluctuation in the number of marine teleosts can be assigned to three broad types. Type I: unstable and unpredictable Subtype IA: irregular and short-spaced e.g. Pacific saury and Pacific sandlance Subtype IB: large-scale and cyclical e.g. sardines and herrings Type II: stable and predictable e.g. tunas and flatfishes In what environments did these three types have evolved? Cushing (1975) summarized the longitudinal features of production in the ocean. In the higher latitudes, while the large-scale primary production occurs in a short time and the productivity is very high, the delay period between the trophic levels is long, resulting in low ecological efficiency. In other words, productivity is high, efficiency is low, and variability is large. On this occasion, the abundance of a few species large in biomass largely varies corresponding to the fluctuation in niche size and they alternate with one another at shorter intervals, illustrating a phenomenon known as “alternation between species”. The structure of such a community is simple and the relation among species is lax. On the other hand, while in the low latitudes the low and continuous primary production occurs, the delay period between the trophic levels is short, resulting in a high ecological efficiency. In this case, while the productivity is low, the efficiency is high. There are many species whose biomasses are small and remain stable. The structure of a community is complex and interspecific relations are keen. 494 Not only the high latitudes are productive in the ocean.