Populations of marine fishes fluctuate in abundance due to both climatic events and to the effects of fishing. Some stocks have been shown to fluctuate in abundance over centuries (Cushing 1982, Soutar and Isaacs 1969), while others show variation in recruitment, of an order of magnitude, over a few years (Cushing 1975, 1982, Kawasaki et al. 1991). Climatic events can produce both short-term and long-term changes in fish stocks. Cushing (1975) has suggested that the link between climate and production is due to the match or mismatch in production of fish larvae and their food, especially in temperate waters where fish tend to spawn at fixed periods while the food production cycle varies.
Temporal replacement of one species by another has occurred within fisheries. In the western English Channel during the late 1920s and 1930s the herring population declined and was replaced by the pilchard (Cushing 1975). There is no evidence that these changes were due to fishing, rather they are associated with a rise in sea temperature which led to a replacement of cooler water species by warmer water species of teleosts and invertebrates (Southward 1963). Off California stocks of sardine declined during the 1950s and were replaced by anchovies (Ahlstrom 1966). This decline is linked with change in temperature but was enhanced by fishing (Beverton 1990, Cushing 1982). Fish scales preserved in anaerobic sediments off California have provided a chronological record of the abundance of pelagic fishes which show fluctuations in abundance of sardines and anchovies over hundreds of years (Soutar and Isaacs 1969).
There is increasing evidence for synchronous changes in fish stocks at the global level. Changes have occurred in widely separated stocks in spite of different management practices between regions (Maan 1993). For example catches of sardines in the Peruvian and Californian upwelling fisheries have varied synchronously with sardine catches off South-east Asia over the past 80 years (Maan 1993). El Niño events in the Pacific produce low catches of sardine off Peru and high catches of jack mackerel off Tasmania (Maan 1993). These observations provide strong evidence that large scale climatic events have a major influence on the abundance of fish stocks so that monitoring of climatic events may provide a predictive tool for quota setting (Maan 1993).
Many fisheries have shown a successional pattern of exploitation: the collapse of single species followed by exploitation of other species in time or area. The Norwegian pelagic fleet moved from herring to capelin to mackerel during the 1960s (Garrod 1973). In the North Sea fisheries landings were relatively constant at 1–1.5 million tonnes per annum for 50 years, but increased rapidly in the 1960s with the development of industrial fisheries, firstly on juvenile herring and mackerel and later on Norway pout, sand-eel and sprat (Hempel 1978). As sole stocks were depleted fishers moved to other areas (Garrod 1973). This pattern of replacement has not been due to an increase in abundance of secondary species, but to a change in fishing targets following over exploitation of the primary species (Daan 1980). In Pacific salmon fisheries some minor stocks have disappeared with heavy exploitation (Loftus 1976). In the Irish Sea the common skate Raia batis was once common but is now very rare. The species is slow growing, with a high age at first maturity and low fecundity, in comparison with demersal teleosts which are the target species in the Irish Sea trawl fisheries (Brander 1981).
Pelagic fish appear to be less resilient to long-term climatic changes than demersal species due to their lower fecundity (Cushing 1975) and shorter life histories (Maan 1993). In addition their schooling behaviour makes them more vulnerable to industrial fishing methods (Rothschild 1986) and several species have collapsed (shown a rapid and dramatic decline in abundance) in the past 50 years. Fishing has been the main cause of collapse in stocks of the Atlantic herring, the Pacific herring, the Pacific sardine, Japanese sardine, South Atlantic pilchard, Peruvian anchovy, Barents Sea capelin, and Pacific mackerel (Beverton 1990). In nine out of ten cases collapsed stocks have recovered when fishing has been stopped or cutback, with a tendency for those stocks with the most severe decline to show the slowest recovery. Only the Icelandic spring spawning herring stock, which collapsed in the late 1960s and early 1970s, has shown no sign of recovery to date (Beverton 1990).
The “gadoid outburst” (Cushing 1984) and associated rise of industrial fisheries in the North Sea may be attributed in part to the decline in herring stocks (Beverton 1990) which released food for gadoid recruits, but the rise in gadoid stocks originated from natural changes in the ecosystem. The spring production of the zooplankton Calanus, the principal diet of cod larvae, was delayed about one month in response to cooler sea temperatures in the 1960s and 1970s, providing a better match between food density and cod larvae, which are produced at a fixed spawning season (Cushing 1984).
Major changes in biomass and species composition has occurred in the Northwest Atlantic demersal fisheries over the past 30 years (Fogarty 1992). The Georges Bank fishery changed from one dominated by gadoids in 1963 to one dominated by dogfish in 1986, a change brought about by high fishing mortalities on cod and haddock and an increase in abundance of dogfish (Sissenwine and Cohen 1993).
In a typical response to fishing a stock will show a decrease in average age as the older and larger fish are removed, leaving a higher proportion of faster growing young fish; this “fishing up” effect was first described by Baranov (1918). Growth in fish is in part density dependent and so when the biomass is reduced through fishing the growth rate may increase and, under moderate exploitation, even lead to an increase in average size at age. For example in the southern Baltic stock of flounder Platichthys flesus the average age of fish and the catch rates declined as the fishery expanded between 1905–25, but the average length and weight of fish increased (Kandler 1932). However, by 1931 the average length had started to decrease as the increase in growth rate was unable to compensate for the rate of removal of fish from the population (Kandler 1932). This fishing up effect was viewed positively as it improved the economic value of the stock (Kandler 1932). In the plaice Hippoglossoides platessoides fishery on the Grand Banks the average size at age increased between the 1950s to 1970s even though catch rates fell over the same period in response to an increase in exploitation (Pitt 1975). There were no significant trends in temperature over the period that may have affected growth rates, but a reduction of plaice density and of competitors through heavy fishing made more food available for plaice (Pitt 1975).
The development of a theory of fishing is accredited to Baranov in the Russian Federation and to Thompson in the USA. This area of population dynamics was expanded and developed by many others to become the core of fisheries science. The management models are based on the concept of surplus production and the maximum sustainable yield. Populations are assumed to be in equilibrium and reach an optimum size in accordance with the carrying capacity of the environment. When the population is reduced through fishing, then recruitment and growth increase to return the population to equilibrium. The compensatory increase in growth and recruitment in response to exploitation is common and widely reported in the fisheries literature. These compensatory changes have masked genetic changes in life history characters due to exploitation and made genetic effects difficult to detect.
