Long-term trends in ten non-target North Sea fish species

ICES Journal of Marine Science, 53: 1063–1078. 1996 Long-term trends in ten non-target North Sea fish species Henk J. L. Heessen and Niels Daan Heessen, H. J. L., and Daan, N. 1996. Long-term trends in ten non-target North Sea fish species. – ICES Journal of Marine Science, 53: 1063–1078. Catch data on 10 non-target fish species from the International Bottom Trawl Survey during the years 1970–1993 are analysed for changes in distribution and abundance by size class. Trends in catch rates of spurdog, starry ray, bib, poor cod, four-bearded rockling, grey gurnard, bullrout, long rough dab, dab, and lemon sole have been compared using correlation and cluster analysis with indices describing different aspects of the North Sea ecosystem, including biomass of pelagic, demersal and industrial species, temperature, eutrophication, and beam trawl effort. Most species appear to have increased over the period. However, the statistical analysis does not provide a plausible explanation of the factors responsible for the observed changes. ? 1996 International Council for the Exploration of the Sea Key words: abundance, distribution, long-term changes, non-target species, North Sea. H. J. L. Heessen and N. Daan: Netherlands Institute for Fisheries Research, PO Box 68, 1970 AB IJmuiden, The Netherlands. Introduction Demersal fisheries in the North Sea are associated with large by-catches of non-target fish species and invertebrates, most of which are discarded at sea. Fisheries obviously affect abundance of these species. However, possible changes in abundance might also have been induced by other anthropogenic factors (eutrophication, pollution, greenhouse effects), or by natural changes in the biotic or abiotic environment. Data on long-term changes in distribution and abundance present a basis for evaluating the interaction between fisheries, other human activities, and natural processes. For non-target species the source of time-series information is generally limited to research vessel surveys (e.g. Richards et al., 1978; Knijn et al., 1993; van Leeuwen et al., 1994), but as yet the available data have not been fully utilized. One of the longest North Sea time series for demersal species is provided by the ICES-coordinated International Bottom Trawl Survey (IBTS), which has been carried out annually in February since 1965. Of the most abundant non-target species caught (cf. Knijn et al., 1993) 10 species have been selected for analysing changes in population parameters in relation to available information on trends in possible causal factors. The selected species consist of spurdog (Squalus acanthias), starry ray (Raja radiata), bib (Trisopterus luscus), poor cod (Trisopterus minutus) four-bearded rockling (Rhinonemus cimbrius), grey gurnard (Eutrigla gurnardus), bullrout (Myoxocephalus scorpius), long rough dab (Hippoglossoides platessoides), dab (Limanda limanda), 1054–3139/96/061063+16 $18.00/0 and lemon sole (Microstomus kitt). Relatively little is known about the biology of many of these species, but a summary can be found in Knijn et al. (1993). Changes over time might be expected in the size composition, the distribution, and abundance of individual species and therefore these aspects are considered. Because factors affecting abundance often operate at a regional scale (e.g. fisheries, eutrophication), variations in abundance are studied by area. Material and methods The IBTS started off as a young herring survey in 1965, when sampling was restricted to the southern and central North Sea. The survey area was gradually extended northwards and eastwards and from 1974 onwards included the entire North Sea, Skagerrak, and Kattegat. Over the years, the survey evolved to a highly standardized, internationally co-ordinated general trawl survey, in which nine countries have been participating. Full technical details are given in ICES (1992). Although commercially important fish species have been the principal target, length data of all by-catch species have routinely been collected by most participants. Stratification of hauls is based on the grid of ICES statistical rectangles. The ICES Roundfish Areas (Fig. 1) have been used here to investigate spatial differences in trends of abundance. Data are stored in the ICES IBTS Database in Copenhagen, but to date only data collected since 1983 are completely computerized. For the period 1970–1982, ? 1996 International Council for the Exploration of the Sea 1064 H. J. L. Heessen and N. Daan E5 E6 E7 E8 E9 F0 F1 F2 F3 F4 F5 F6 F7 F8 F9 G0 G1 G2 52 51 Roundfish areas 50 1 49 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32 31 30 29 28 27 61°N 60° 59° 3 8 2 58° 57° 7 56° 55° 4 6 54° 53° 5 52° 51° 50° 4°W 2° 0° 2° 4° 6° 8° 10° 12°E Figure 1. Subdivision of the North Sea in ICES ‘‘Roundfish Areas’’. 40 1 large gaps exist in the available data, because several countries have not yet submitted the computerized information. This causes problems when analysing trends in abundance, because some areas are inadequately covered by the reported catches during the early years. The number of rectangles for which data are available by area (Fig. 2) indicates that only areas 2 and 6 are well covered by data in most years. Length-frequency distributions (LFD) were plotted by species to reveal indications of the constituting age classes. However, even the average LFDs (Fig. 3) for the period 1983–1993 are often variable or show merely one mode. Nevertheless, this information was used for splitting catches into juvenile and older components, which were treated separately in the analysis of distribution and abundance. Both spurdog and starry ray exhibit unusual LFDs skewed to the left. Based on maturity data (Knijn et al., 1993), the dividing line between immature and mature dogfish was set at 65 cm. For distinguishing between small and large starry ray, a line has been drawn arbitrarily at 20 cm. LFDs for bib and poor cod show peaks at 15–20 cm and at 11 cm, respectively. According to information from otolith samples taken in February 40 5 30 30 20 20 10 10 0 70 72 74 76 78 80 82 84 86 88 90 92 0 70 72 74 76 78 80 82 84 86 88 90 92 40 2 40 6 30 30 20 20 10 10 0 70 72 74 76 78 80 82 84 86 88 90 92 0 70 72 74 76 78 80 82 84 86 88 90 92 40 3 40 7 30 30 20 20 10 10 0 70 72 74 76 78 80 82 84 86 88 90 92 0 70 72 74 76 78 80 82 84 86 88 90 92 40 4 40 8 30 30 20 20 10 10 0 70 72 74 76 78 80 82 84 86 88 90 92 0 70 72 74 76 78 80 82 84 86 88 90 92 Figure 2. Number of rectangles for which data are available 1970–1993. 1–8 refer to areas shown in Fig. 1. Non-target North Sea fish species 20 Squalus acanthias 15 15 1065 Eutrigla gurnardus 10 10 5 5 0 20 40 60 10 80 100 120 Raja radiata 0 10 15 20 30 40 50 Myoxocephalus scorpius 10 5 5 0 10 20 30 15 40 50 60 Trisopterus luscus 0 10 10 5 5 0 10 15 20 30 40 50 Trisopterus minutus 0 10 5 (...truncated)