The objective of this pilot project is to study growth patterns in 'fossil' and present day otolith fish in order to study the compensatory response of populations to exploitation.
Compliance: The project addresses research task 5.1.1 ("environmental factors on fish and fisheries") and task 5.2.1. ("effects of fisheries on the environment") and focuses on changes in somatic growth of several demersal North Sea fish species. The analysis of changes in growth may provide relevant information to assess the level of compensatory response of somatic growth to changes in population size through exploitation.
To create a sound scientific basis for the sustainable exploitation of fisheries resources, knowledge of the vital parameters affecting population dynamics is important (Beverton & Holt, 1957). Growth is one of these vital parameters and changes in growth have a profound impact on the productivity of the resource. This is due to the increase in weight of the exploited biomass, but also to its effect on the reproductive output. Both maturation and egg production are being affected by the growth (Rothschild, 1986; Rijnsdorp, 1990). The increase in haddock landings in the 1960s has been (partly) ascribed to the increase in growth (Jones & Hislop, 1978). similarly, an increase in flatfish landings were related to the observed increase in growth (Bannister, 1978; de Veen, 1978; Rijnsdorp & van Beek, 1991), whereas the decrease in landings of plaice in the 1980s coincided with a decrease in growth rate (Anon., 1996).
The present proposal attempts to answer the question whether density dependent growth has been a regular phenomenon in unexploited or lightly exploited marine demersal fish, and to investigate whether its role is related to the trophic position of a fish species within the food web. Based on studies of heavily exploited populations, the general pattern emerges that growth during the adult phasewls not,Bensitv-dependent, whereas growth in the juvenile phase may be density-dependent (Valiela, 1984; Daan et al., 1991; van der Veer et al., 1994).The question arises as to whether this pattern holds for unexploited or lightly exploited populations. There is some evidence suggesting that density-dependent processes during both the adult and juvenile phase may become more pronounced at reduced levels of exploitation'. An increase in growth in several flatfish species has been observed since the turn of the century, which may reflect the release from density Dependent regulation due to increased exploitation (Rijnsdorp, 1994). Also a decrease in adult growth rate of North Sea plaice was observed that coincided with the build up of biomass during the period of reduced fishing during World War II (Beverton and Holt, 1957; Rijnsdorp and van Leeuwen, 1992).
On theoretical grounds we expect the compensatory response to differ among species in relation to the position of the species, or life history stage of the species, within the food web. For instance, Gislason and Rice (1996) and Rice and Gislason (1996) showed that an increase in exploitation resulted in a decrease of the larger, exploited, size classes, but in an increase of the smaller size classes due to a reduction in predation pressure. Differences in compensatory response are also expected for those species which utilise sheltered nursery grounds.
The study of the compensatory response of marine fish species to heavy exploitation has been severely hampered by the lack of data from the preindustrialised fishing era (Pope and Macer, 1996; Rijnsdorp and Millner, 1996). A new avenue, however, may be opened by bringing together the disciplines of fisheries biology and archaeology. Among archaeologists there is a growing interest to analyse fish remains from excavations. As a result of the systematic use of sieves during archaeological excavations, the number of fish remains available for study has increased drastically during the last two decades (Wheeler & Jones, 1989; Van Neer & Ervynck, 1993). Many archaeological sites dated to the mediaeval period have yielded thousands of fish remains although otoliths are usually poorly represented due to the unstable nature of the otoliths (made of aragonite) if compared to fish bones (made of hydroxyapatite). Growth increment studies have thus far mainly concentrated on the season of death of the individual fish (Brewer, 1987; Carlson, 1988; Wilkinson, 1981; Monks & Johnston, 1993; Van Neer, 1993). Because growth rate is not directly relevant to the archaeological interpretation of sites, this topic has thus far been under exploited. A survey of the literature demonstrates that such studies have only been undertaken for Nile perch (Lates niloticus) on a Neolithic site in Egypt (on the basis of vertebrae; Brewer, 1992) and for the Atlantic croaker (Micropogonias undulatus) from North American sites dated between 1500 BC and 1800 AD (based ototoliths; Hales & Reitz, 1992). Information on growth rate, however, may enhance the archaeological interpretation with regard to the origin of the fish.
Because of it's pilot character, the project has a limited scope in terms of participation, number of species studied and the required resources. If success full, a more extensive collection of fish remains, including otoliths of other fish species is available for further studies. The project is expected to highlight the methodological potential, and to provide important, information on the effects of exploitation on growth of marine fish species, which is essential to asses the effects of increased exploitation since the industrial revolution which is relevant to evaluate the possible effects of reductions in fishing pressure implicit in the Common Fishery Policy.
Funding SchemeCSC - Cost-sharing contracts