Instability of harvested populations environmental fluctuations

It is tempting to attribute all fisheries' collapses simply to overfishing and human greed. Doing so, however, would be an unhelpful oversimplification. There is no doubt that fishing pressure often exerts a great strain on the ability of natural populations to sustain levels of recruitment that counteract overall rates of loss. But the immediate cause of a collapse - in 1 year rather than any other - is often the occurrence of unusually unfavorable environmental conditions. Moreover, when this is the case, the population is more likely to recover (once conditions have returned to a more favorable state) than it would be if the crash was the result of overfishing alone.

The Peruvian anchovy (see Figure 15.8), prior to its major collapse from 1972 to 1973, had already suffered a dip in the upward rise in catches in the mid-1960s as a result of an 'El Niño event': the incursion of warm tropical water from the north severely reducing ocean upwelling, and hence productivity, within the cold Peruvian current coming from the south (see Section 2.4.1). By 1973, however, because fishing intensity had so greatly increased, the effects of a subsequent El Niño event were much more severe. Moreover, whilst the fishery showed some signs of recovery from 1973 to 1982, in spite of largely unabated fishing pressure, a further collapse occurred in 1983 associated with yet another El Niño event. Clearly, it is unlikely that the consequences of these natural perturbations to the usual patterns of current flow would have been so severe if the anchovy had not been exploited or had been only lightly fished. It is equally clear, though, that the history of the Peruvian anchovy fishery cannot be understood properly in terms simply of fishing, as opposed to natural events.

The three Norwegian and Icelandic herring fisheries also collapsed in the early 1970s and had certainly been subjected to increasing fishing intensities prior to that. Once again, however, an oceanic anomaly is implicated (Beverton, 1993). In the mid-1960s, a mass of cold, low-salinity water from the Arctic Basin formed north of Iceland. It drifted south until it became entrained in the Gulf Stream several years later, and then moved north again - although well to the east of its southward track. It eventually disappeared off Norway in 1982 (Figure 15.12a). Data for the number of 'recruits per spawner', essentially the birth rate, are illustrated in Figure 15.12b for the Norwegian springspawning and the Icelandic spring- and summer-spawning herring between 1947 and 1990, in terms of the difference each year between that year's value and the overall average. Also illustrated are the corresponding yearly temperature differentials in the Norwegian Sea, reflecting the southward and northward passage of the anomalous cold water body. There was a good correspondence between the cold water and poor recruitment in both the Icelandic and Norwegian stocks in the late 1960s and in the Norwegian stocks in 1979-81, the Icelandic stocks being then extinct (spring spawners) or too far west. It seems likely that the anomalous cold water led to unusually low recruitment, which was strongly instrumental in the crashes experienced by each of these fisheries.

This cannot, however, account for all the details in Figure 15.12b - especially the succession of poor recruitment years in the Norwegian stocks in the 1980s. For this, a more complex explanation is required, probably involving other species of fish and perhaps alternative stable states (Beverton, 1993). None the less, it remains clear that whilst the dangers of overfishing should not be denied, these must be seen within the context of marked and often unpredictable natural variations. Given the likely effects of environmental conditions on the vital rates of harvested populations, a reliance on models with constant vital rates is even more risky. Engen et al. (1997) argue that the best harvesting strategies for such highly variable populations involve constant escapement (see Section 15.3.4).

harvesting operations with multiple equilibria are susceptible to dramatic irreversible crashes the anchoveta and the El Niño herring and cold water

Figure 15.12 (a) The track of a large mass of cold, low-salinity water in the 1960s and 1970s, showing its presence in the Norwegian Sea both in the mid-1960s and the period 1977-82. (b) Annual differentials between overall averages and ln (recruits per spawner) for three herring stocks in the Norwegian Sea, and (c) the temperature in the Norwegian Sea. The Icelandic spring-spawning stock never recovered from its collapse in the early 1970s, preceded by low recruitment in the 1960s. (After Beverton, 1993.)

Figure 15.12 (a) The track of a large mass of cold, low-salinity water in the 1960s and 1970s, showing its presence in the Norwegian Sea both in the mid-1960s and the period 1977-82. (b) Annual differentials between overall averages and ln (recruits per spawner) for three herring stocks in the Norwegian Sea, and (c) the temperature in the Norwegian Sea. The Icelandic spring-spawning stock never recovered from its collapse in the early 1970s, preceded by low recruitment in the 1960s. (After Beverton, 1993.)

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