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Hypothetical underlying processes

Climate-induced fluctuations in fish catches have been demonstrated for several

commercial fish stocks (e.g. Cushing, 1995; Cushing and Dickson, 1976). Fluctuations

in temperature have been shown to influence: (1) the catchability (i.e., the fraction of a

stock which is caught by a standardized unit of fishing effort), or in other words, the

probability of capture, (2) biological processes, especially recruitment and growth of

fish, and (3) the spatial dynamics of migrating fish populations (e.g. Corten, 1990;

2001; Southward et al., 1988).

A trap is a passive gear that stops mature BFT migrating along the shore and

guides them to a final enclosure, where fishermen finally gaff them. Local variations in

environmental conditions could affect catchability, through the success of the fishing

operation and/or the fish behaviour (e.g. tuna migrating more or less offshore).

However, such a process is unlikely to explain simultaneous long-term fluctuations in

BFT catches between distant traps (that spread over more than 2000 km, for more

details see Ravier and Fromentin, 2001).

Long-term fluctuations in BFT traps catches could be more easily explained by

hypotheses (2) and (3) (see also Fromentin and Fonteneau, 2001). Temperature is

known to affect fish recruitment and growth directly, through the daily development and

mortality of early stages (Pepin, 1991; Ottersen and Sundby, 1995) or indirectly, e.g.,

through changes in food supply for fish larvae and juvenile (Cushing, 1995). Fromentin

(2002b) further showed, through simulation modelling, that stochastic variations in

recruitment can lead to long-term fluctuations in the spawning stock biomass (SSB),

due to the sum of the time delayed noise terms (this process being close to that known

as resonant effects, see Bjørnstad et al., 1999). Thus, both long-term and short-term

variations in temperature could, through their effects on recruitment and/or growth, lead

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