Time-series data for a selection of forty fish species caught during the International Bottom Trawl Survey

ICES Journal of Marine Science, 53: 1079–1084. 1996 Short communication Time-series data for a selection of forty fish species caught during the International Bottom Trawl Survey Henk J. L. Heessen Heessen, H. J. L. 1996. Time-series data for a selection of forty fish species caught during the International Bottom Trawl Survey. – ICES Journal of Marine Science, 53: 1079–1084. Relative abundance estimates of 40 fish species caught during the ICES coordinated International Bottom Trawl Survey are presented for the period 1970–1993. Trends in survey catches for eight commercial species are compared with total stock biomass estimates derived from stock assessment. In general, there is a remarkably good correspondence between these two independent parameters, suggesting that the survey is capable of registering significant changes in abundance. Among the 40 species, there are many more showing a positive trend during recent years than a negative one. However, the commercial species are all either declining or are stable. The most significant changes are observed in ‘‘southern’’ species, which is probably related to a period of relatively warm winters after 1990. In many resident species, however, that appear to be increasing over a prolonged period of time, this explanation does not seem to apply. ? 1996 International Council for the Exploration of the Sea Key words: fish, long-term trends, North Sea, trawl survey. Henk J. L. Heessen: Netherlands Institute for Fisheries Research, PO Box 68, 1970 AB IJmuiden, The Netherlands. Introduction Materials and methods The data from the ICES coordinated International Bottom Trawl Survey (IBTS) provide one of the longest time series available for demersal fish species in the North Sea. The survey has been carried out annually in February since 1965. Initially, herring (Clupea harengus) was the target species, and the survey coverage was restricted to the southern and central North Sea. Since 1969, the Skagerrak and Kattegat have been sampled and from 1974 onwards the entire North Sea area has been included in the survey area. An overview of the distribution of all fish species caught during the IBTS, based on catches in the years 1985–1987, is given in Knijn et al. (1993). Time series for 10 of the most abundant non-commercial species caught are presented in Heessen and Daan (1996), and for 6 species of rays in Walker and Heessen (1996). This paper presents time series of average catch rates for a selection of 40 other species for the period 1970–1993, including 8 commercially important species. The survey catches of the latter are compared with estimates of total stock biomass (TSB) (from ICES, 1995a). Survey methods are summarized in Heessen and Daan (1996) and a full description is available in ICES (1992). All data are stored in the IBTS Database at the ICES Secretariat in Copenhagen. Data on by-catch species are incomplete for the period 1970–1982, because not all data have been computerized. For eight commercial species the survey catches are complete. Average catch rates in numbers per hour fishing have been calculated over all hauls made in the North Sea proper (cf. areas 1–7 in Fig. 1 of Heessen and Daan, 1996). 1054–3139/96/061079+06 $18.00/0 ? 1996 International Council for the Exploration of the Sea Results Sharks (Fig. 1) Except for spurdog (Squalus acanthias) (see Heessen and Daan, 1996), sharks are caught infrequently. Tope (Galeorhinus galeus) has only been reported since the 1980s. This species is probably mainly a summer visitor in the German Bight (Knijn et al., 1993). Smooth hound (Mustelus mustelus) is caught in very low numbers. 1080 H. J. L. Heessen Scyliorhinus caniculus N/hr 0.8 8000 0.6 6000 0.4 4000 0.2 2000 0.0 70 75 80 85 0 90 70 75 8000 0.02 6000 85 90 4000 0.01 2000 70 75 80 85 0 90 70 75 80 85 90 Engraulis encrasicolus Mustelus mustelus 0.05 0.04 0.03 0.02 0.01 0.00 80 5000 4000 3000 2000 35 0 Sprattus sprattus Galeorhinus galeus 0.05 0.00 TSB Clupea harengus 3 2 1 70 75 80 85 0 90 Figure 1. Average catch per one hour fishing for three shark species, 1970–1993. 50 40 30 20 10 0 The gadoid catch is largely dominated by the four commercial species cod (Gadus morhua), haddock (Melanogrammus aeglefinus), whiting (Merlangius merlangus), and Norway pout (Trisopterus esmarki). After a number of strong year classes in the 1970s the abundance of cod gradually decreased. Since around 1975, both survey catches and TSB show the same downward trend. Catches of haddock and whiting 80 85 90 70 75 80 85 90 Argentina silus 0.4 Pelagic species (Fig. 2) Gadoids (Fig. 3) 75 Argentina sphyraena Lesser-spotted dogfish (Scyliorhinus caniculus) is slightly more abundant, particularly in the north-western parts. Since the early 1980s the catches have been fairly stable. Both the survey catches and TSB clearly show the reduction in herring abundance in the late 1970s, and the recovery of the stock in the following 10 years. Catches of sprat (Sprattus sprattus) are rather variable. Anchovy (Engraulis encrasicolus), a southern species, was regularly caught in the 1970s. Over recent years numbers have been slightly increasing (see also Boddeke, 1996). The lesser argentine (Argentina sphyraena) dominate over the greater argentine (A. silus) in the catches, but both species appear to have increased over the last decade. 70 0.3 0.2 0.1 0.0 70 75 80 85 90 Figure 2. Average catch per one hour fishing for five pelagic species. For herring, the drawn line represents estimated TSB (Total Stock Biomass in ’000 t; right-hand axis; Anon., 1995a). varied considerably over the 24-year period, but recent numbers per hour were at the same level as in the 1970s. For haddock, survey catches and TSB follow the same pattern. For whiting, however, there is no agreement between the two. Since the mid-1980s, the IBTS data show a gradual increase, whereas according to the assessment TSB remains stable (ICES, 1995b). For both saithe (Pollachius virens) and Norway pout (Trisopterus esmarki) there is little resemblance between the survey catches and TSB. Pollack (Pollachius pollachius), tusk (Brosme brosme), ling (Molva molva), and hake (Merluccius merluccius) are caught irregularly without clear trends. Time-series data for a selection of forty fish species Gadus morhua 250 200 150 100 50 0 Trisopterus esmarki 1200 12 000 1600 800 8000 1200 400 4000 800 70 75 80 85 90 0 0 400 70 75 Melanogrammus aeglefinus 2000 1500 1000 500 0 70 75 80 85 90 3000 2000 1000 70 75 80 85 90 80 85 90 0 Brosme brosme 2500 2000 1500 1000 500 0 0.3 1200 3 800 2 400 1 0 0 0.2 0.1 0.0 70 75 Merlangius merlangus 4000 0 1081 80 85 90 Molva molva 70 Pollachius virens 75 80 85 90 Merluccius merluccius 100 1500 75 1000 50 2.0 1.5 1.0 25 500 0 0 70 75 80 85 90 0.5 0.0 70 75 80 85 90 Pollachius pollachius 4 3 2 1 (...truncated)