Life history patterns in pelagic fishes
Pelagics are by definition open ocean fishes throughout their lives. Two general patterns characterize the overall life histories of pelagic fishes, brought on by the relationship of parental versus larval food requirements, life span, spawning frequency, oceanic currents, and fish mobility. These patterns are referred to as cyclonic or anticyclonic.
Cyclonic patterns characterize higher latitude species such as Atlantic Herring, in which the adults and larvae live in different parts of the ocean. Adults have a seasonal feeding area and tend to spawn once per year. Before they spawn, they migrate upcurrent to a region where food for larvae and juveniles will be particularly abundant. Larvae and, later, juveniles drift with the currents to the adult feeding region. These fish invest considerable energy into each spawning episode, both in terms of the costs of the migration and also in egg production. Because of the spatial separation of adult and larval habitats, adults may not have reliable cues for predicting conditions at the spawning grounds, which leads to highly variable spawning success and large fluctuations in year class strength (see Population dynamics and regulation).
Anticyclonic patterns are more characteristic of lowlatitude species such as tropical tunas and scads. The comparative aseasonality of tropical waters leads to less temporal fluctuation but extreme spatial variation in productivity. Adults move in a roughly annual loop through a major ocean basin, during which time they spawn repeatedly (with the exception of Bluefin Tuna) rather than only in particular locales. Larvae and juveniles develop and feed along with adults, carried by the same current system in their relatively nomadic existence. The energy put into reproduction is spread out amongst several spawning episodes. Adults can use local environmental cues to determine the appropriateness of conditions for larvae, which is critical given the low productivity and patchiness of tropical open oceans. Hence anticyclonic species often show weaker fluctuations in year class strength. Within families, tropical species mature more quickly and live shorter lives.
Interestingly, tunas evolved in the tropics but some species such as the Giant Bluefin spend a large part of the year feeding in productive temperate locales (see Block & Stevens 2001). Bluefin show the phylogenetic constraint of their tropical history by returning to the tropical waters of the Gulf of Mexico or Mediterranean Sea to spawn, forcing them into what is more of a cyclonic than an anticyclonic pattern (Rivas 1978). The same historical factors constrain anguillid eels such as the American, European, and Japanese species, which also return from temperate feeding locales to tropical breeding locales, but several years pass between the two life history stages (see Representative life histories of migratory fishes). The high but periodic productivity of small planktonic animals in the open sea and the presence of major ocean currents have been contributing factors in the evolution of dispersive, planktonic larvae in most marine fishes, regardless of whether the adults are planktonic, pelagic, demersal, deep sea, or inshore.
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