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Stable ocean hypothesis
The stable ocean hypothesis (SOH) is one of several hypotheses within larval fish ecology that attempt to explain recruitment variability (Figure 1;〔Houde, E. 2008. Emerging from Hjort’s shadow. Journal of Northwestern Atlantic Fisheries Science 41:53-70.〕 Table 1). The SOH is the notion that favorable and somewhat stable physical and biological ocean conditions, such as the flow of currents and food availability, are important to the survival of young fish larvae and their future recruitment. In the presence of stable ocean conditions, concentrations of prey form in stratified ocean layers; more specifically, stable ocean conditions refer to “calm periods in upwelling ecosystems (sometimes called 'Lasker events')” that cause the water column to become vertically stratified.〔 The concept is that these strata concentrate both fish larvae and plankton, which results an increase of the fish larvae feeding because of the density-dependent increase in predator-prey interactions. Lasker is attributed with constructing this hypothesis in the late 1970s 〔Lasker, R. 1981. The role of a stable ocean in larval fish survival and subsequent recruitment. Marine fish larvae, morphology, ecology and relation to fisheries, p. 80-87. University Washington Press.〕 by building on previous larval fish research and conducting his own experiments.〔Lasker, R., H. M. Feder, G. H. Theilacker, and R. C. May. 1970. Feeding, growth, and survival of Engraulis mordax larvae reared in the laboratory. Marine Biol. 5:345-353.〕〔Lasker, R. 1975. Field criteria for survival of anchovy larvae: the relation between inshore chlorophyll maximum layers and successful first feeding. Fish. Bull., U.S. 73:453-678.〕〔Lasker, R., and J. R. Zweifel. 1978. Growth and survival of first-feeding northern anchovy (Engraulis mordax) in patches containing different proportions of large and small prey, p.329-354. In Spatial Pattern in Plankton Communities, (Ed. J. H. Steele), Plenum New York, 470 p.〕 He based the SOH on case studies of clupeid population fluctuations and larval experimentation.
==Case study evidence==
To support this hypothesis, Reuben Lasker cited the disconnect between spawning stock biomass and the recruitment of numerous fish species.〔Dippner, J. W. 1997. Recruitment success of different fish stocks in the North Sea in relation to climate variability. Deutsche Hydrgraphische Zeitschrift 49 (2-3): 277-293.〕 One explanation of this disconnect suggests larval recruitment is influenced by spatial and temporal patterns of their food, like phytoplankton or zooplankton, which can be greatly affected by ocean currents and mixing.〔Haury, L. R., McGowan, J. A., and Wiebe, P. H. 1978. Patterns and processes in the time-space scales of plankton distribution. Spatial Pattern in Plankton Communities (ed. J. H. Steele), p. 277-327. Plenum, New York.〕 In his publication Marine fish larvae: Morphology, ecology, and relation to fisheries (1981), he points out, for example, the Peruvian anchovy fishery collapse that resulted from a dramatic decrease in population size during the early 1970s.〔Valdivia, G., J. E. 1978. The anchoveta and El Niño. Rapp. P. –v. Réun. Cons. Int. Explor. Mer 173:196-202.〕 Officials and researchers from the Peruvian government and United Nations Food and Agriculture Organization submitted that the causal factors were a combination of strong fishing pressure and weak year classes that resulted in insufficient reproduction and recruitment to support the fishery.〔Murphy, G. T. (chairman). 1974. Report of the fourth session of the panel of experts on stock assessment on Peruvian anchoveta. Instituto del Mar del Peru (Callao), Boletin 2:605-719.〕 This explanation seemed to explain the diminished population trends of similar species from other regions, including the Pacific and Japanese sardines and the Atlanto-Scandian herring.〔
Lasker, however, opposed this conclusion while citing the seemingly miraculous recovery of the troubled Japanese sardine population from scarcity (e.g. thousands of landed tons) to prominent abundance (e.g. more than a million landed tons).〔 Another researcher studying the rebound of the Japanese sardine, Kondo (1980), identified an unusually strong 1972-year class, which produced successful recruitments in the years that followed. Kondo also noted altered ocean current patterns that increased zooplankton availability in spatiotemporal coincidence with the hatching of the sardine larvae.〔〔Kondo, K. 1980. The recovery of the Japanese Sardine – the biological basis of stock-size fluctuations. Rapp. P.-v. Reun. Cons. Int. Explor. Mer. 177:322-354.〕 The result was increased larval survival and the eventual rebound of the population.〔 Thus, the observed trend is that strong year class anomalies can have major impacts on population sizes and their future stability and growth.〔〔〔Hjort, J. 1926. Fluctuations in the year classes of important food fishes. Cons. Perm. Int. Explor. Mer, Journ. Du Cons., 1:5-38.〕 This concept also illustrates how plankton abundance and ocean currents can be driving factors associated with such trends.〔 Clearly, these patterns become important when considering the predictive models necessary to manage and sustain important fisheries and the stocks that support them.
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