The notion that fish species form stocks

The development of the notion that fish species form stocks (or populations) was a gradual process, taking place between 1878 and about 1930. This new concept was due to a shift from "migration thinking" to "population thinking." The relevant research during this period was primarily directed towards the intertwined practical problems of the causes of fluctuations in fisheries and the evaluation of overfishing. The development of three components of the population concept is traced: pattern, richness, and variability. The so-called racial investigations of Heincke were instrumental to the conclusion that most marine fish species comprise geographically distinct spawning populations which may intermingle during other parts of the life cycle migrations. The work of Committee A of ICES under the leadership of Hjort led to the understanding that year-class strengths within populations can be extreme and lead to population variability. The studies by Schmidt between 1917 and 1930 led to the conclusion that there are species-specific differences in numbers of populations, with European eel, for example, having a single population within the distributional limits to species, and eelpout having many populations. The work of Committee B o f ICES on the overfishing problem contributed to the shift to "population thinking" by demonstrating the restricted geographic area of plaice in the North Sea, and the relatively limited abundance estimates of the exploited resource. The change in thinking had profound influences on both the studies on the over-fishing problem (1930-1950s) and the conceptual framework within which the influence of climate variability on fish recruitment has been addressed up to the present time. The research by Heincke and Schmidt had an impact on the broader scientific community, whereas the key contributions by Hjort on year-class variability do not appear to have had any influence on the development of ecological theory.