5 - Natural fluctuations of fish stocks

Summary

In Chapter 2 we discussed the variability of fish catches and fish stocks at some length. Catches from some stocks have risen rapidly and then fallen just as rapidly to almost nothing (the Peruvian anchovy, the California sardine, the Japanese sardine). But after some time, decades in some cases, these stocks have bounced back and sometimes even surpassed their previous peaks. In this chapter we shall take a closer look at variations in fish stocks, in particular some other stocks that have collapsed. There are few if any cases of collapses without subsequent recovery, however, even though it may take a long time, even decades.

The variability of fish stocks can play out on several timescales. The shortest timescale is the year-to-year variability in recruitment – that is, variations in the size of fish cohorts. In temperate or cold waters spawning is seasonal; fish gather in their spawning grounds at a certain time of the year and may migrate hundreds of kilometres to get there (an example is the Northeast Arctic cod migrating from the Barents Sea to the area around the Lofoten islands). Each fish spawns millions of eggs, but most get eaten by other fish before they mature or die for other reasons. Small variations in an enormously high egg mortality could explain the large variations in cohort size. Suppose a fish spawns 70,000 eggs and that the normal mortality is 99.99 per cent. Of the 70,000 eggs only seven would normally survive. With a mortality of “only” 99.98 per cent, fourteen would survive, and the recruitment from that fish would be doubled.

The variability in recruitment can also play out on longer timescales. First, there are the apparently irregular high peaks of recruitment; typically fish cohorts are below average size in terms of numbers, but every once in a while there is an exceptionally large recruitment, which sustains the fishery for many years to come. We shall see an example of this below, drawn from the Northeast Arctic cod stock. The causes of these variations are not well known. The availability of critical food at a critical time has been mentioned.