CEPHALOPOD POPULATIONS - DEFINITION AND DYNAMICS

The study of cephalopod populations currently lacks the means to defin e populations adequately and to resolve basic systematic confusions. Q uantitative data are usually only available from indirect sources such as commercial fisheries and from estimates of consumption by higher p redators. Despite these methodological difficulties it is clear that c ephalopods comprise a major component of biomass globally, throughout all fully marine habitats. Life-cycle characteristics common to the co leoids - early and/or semelparous breeding, rapid growth, short lifesp an, little overlap of generations, vulnerability to predation and envi ronmental variables - result in wide inter-annual fluctuations in abun dance. Most of the pelagic forms also undertake large- or meso-scale m igrations which, coupled to shifting patterns of oceanographic variabl es, contribute to the unpredictability of distribution and density ass ociated with many cephalopod species. Temporal and spatial patterns of breeding, seasonality, growth, recruitment and mortality are clearly evident in most of the better-studied species. But exceptions to patte rn (e.g. variable growth rates, extended breeding, complex recruitment ) also seem to be important intrinsic characteristics. Levels of genet ic variation in cephalopods are relatively low, and their population d ynamics appear to be influenced principally by phenotypic plasticity i n response to environmental variability. In such universally short-liv ed species the maintenance of this diversity balances the risks of mor tality factors combining at any one time to cause periodic local extin ction. The extent and scale of the interactions between cephalopod pop ulations and other trophic levels suggests that major ecological pertu rbations such as environmental shifts, or imposed effects such as comm ercial fishing, whether directed at cephalopods or other species, are likely to have an impact on their populations. As short-lived species with high turnover of generations, plastic growth and reproductive cha racteristics, high mobility and catholic predatory habits, they are al ways poised to respond to changed balances in their environment. Studi es on cephalopod populations have expanded considerably in numbers and scope in the last 25 years, driven by increased interest in and recog nition of their roles in the marine ecology, as well as their increasi ng value as globally exploited resources. Despite these recent advance s, the information and concepts arising from their study is only slowl y entering mainstream biological thought and becoming accommodated in broad-scale models of the marine ecosystem.