5.3.1 Benthos – Fish sampling programme
The GSBTS (German Small-scale Bottom Trawl Survey) includes coupled analyses of demersal fish fauna, epibenthos and hydrography in order to contribute to the physical and biological characterization of habitats of demersal North Sea fish. Within the GSBTS survey, which was initiated in 1987, six areas of the North Sea have been selected for these coupled analyses conducted yearly since 1999 to study the spatial and functional coupling between demersal fish and invertebrate epibenthic fauna (Figure 5.3-1; Ehrich 1988;Callaway et al., 2002, Hinz et al., 2005). The demersal fish fauna is sampled with the same gear as used in the IBTS Survey (GOV, 30-min hauls), while the GSBTS focuses on small-scale analyses to complement the large-scale international survey. Spatial resolution: typically between 20 and 30 hauls within the six 10x10 nautical mile squares, each sampled within three consecutive days. Epibenthos and sediment are sampled in tight spatial and temporal coupling to the GOV hauls, using a 2-m beam trawl and a van-Veen grab, respectively. Accompanying depth profiles of hydrographic parameters are taken. Methods and results of the survey will be presented in a review before the 2007 meeting of WGFE (Ehrich et al., in prep.) and should then be analysed in the framework of essential habitat description.
Figure 5.3-1: Location of sampling areas (“boxes”) within the GSBTS, surveyed in coupled analyses of assemblages of epibenthos and fish.
5.3.2 Benthos–Fish trophic interactions
Fish–benthos relationships have special relevance in feeding habits, and the level of association between these components of the ecosystem will be marked by grade of feeding specializations of each fish species. It is well-known that flatfishes (Pleuronectidae), ventrally blind, except the Greenland halibut (Reinhardtius hippoglossoides), are strongly associated with the bottom relative to most fish species; therefore, changes or disturbances in the benthic habitat may influence their diet composition and condition of these species which could be reflected in their reproductive potential.
In the NAFO area, witch (Glyptocephalus cynoglossus) is the most specialized species, with a diet that consists almost exclusively of benthic prey (Román et al., 2004). There also is a clear dependence on benthos in the cases of American plaice (Hippoglossoides platessoides) and yellowtail flounder (Limanda ferruginea). Other demersal species, as cod (Gadus morhua), wolffishes (Anarhichas sp.) and skates (Rajidae) have a more diverse diet (Rodríguez-Marín et al., 1994; Rodríguez-Marín, 1995; Román et al., 2004) and therefore may be less affected by changes in the benthos.
The data and some results on feeding habits are presented from the Spanish Bottom Trawl Research Survey “Patuxa” in NAFO Area Divs. 3NO in the period from 2002-2005 (Table 5.3-1).
Table 5.3-1: Characteristics of the Spanish Bottom Trawl Research Survey “Platuxa” 2002-2005 (NAFO Area, Divs. 3NO).
DATA OF SPANISH RESEARCH SURVEY PLATUXA
RV YEAR GEAR DATE (DAY/MONTH) DEPTH RANGE (M)
Vizconde de Eza 2002 Campelen 1800 29/04 to 19/05 38 - 1540 Vizconde de Eza 2003 Campelen 1800 11/05 to 02/06 38 - 1666 Vizconde de Eza 2004 Campelen 1800 06/06 to 24/06 43 - 1460 Vizconde de Eza 2005 Campelen 1800 10/06 to 29/06 47 - 1438
The stomach contents of main species in the catch were analyzed on board ship. The results obtained in the study of the diet composition of spotted wolffish (Anarhichas minor), Northern wolffish (Anarhichas denticulatus), Atlantic wolffish (Anarhichas lupus), witch, and yellowtail flounder are summarised below. Characteristics of the sampled individuals are shown in Tables 5.3-2 and 5.3-3, and show the results in weight (%) and number (%) of the main prey groups.
Table 5.3-2: Individuals sampled (NAFO, Div. 3NO, 2002–2005).
Table 5.3-3: Main prey group (% weight and number) by year.
WEIGHT (%) NUMBER (%)
SPECIE PREY GROUP
2002 2003 2004 2005 2002 2003 2004 2005
A. denticulatus Pisces 90.2 23.1 11.3 13.5 9.1 0.5 1.7 0.3
Crustacea 0.7 1.4 5.9 10.3 36.4 2.4 23.9 54.3
Mollusca 2.2 4.5
Echinodermata 2.2 4.1 3.4 1.3 1.1 1.7
Other invertebrates 6.1 69.4 77.6 71.6 40.9 95.4 72.2 43.5
Others 0.8 3.9 1.2 1.3 9.1 0.3 1.1 0.3
A. lupus Pisces 3.4 2.3 1.0 2.9 1.1 1.0 0.1 0.3
Crustacea 2.5 23.3 11.9 14.3 9.5 12.9 13.3 8.5 Mollusca 80.2 37.2 39.4 34.9 62.4 19.6 33.4 59.0 Echinodermata 0.2 3.0 27.4 44.4 5.3 8.6 28.7 28.4 Other invertebrates 10.9 34.1 20.1 3.1 19.0 57.7 23.8 1.8 Other/not identified 2.9 0.0 0.1 0.4 2.7 0.2 0.7 2.0
A. minor Pisces 24.2 34.7 1.9 0.4
Crustacea 4.0 62.5 7.6 4.7 4.5 22.7 8.6 6.7
Mollusca 1.0 0.1 31.9 4.0
LENGTH (CM) NO. INDIVIDUALS SAMPLED SPECIES
Min. Max.
DEPTH RANGE (M)
2002 2003 2004 2005 Total
Anarhichas denticulatus 20 111 57 - 1458 14 39 48 88 189
Anarhichas lupus 6 124 51 - 1337 189 166 150 285 790
Anarhichas minor 11 102 176 - 751 4 9 65 36 114
Glyptocephalus cynoglossus 6 61 43 - 1450 419 581 278 350 1628
Limanda ferruginea 5 60 38 - 156 645 777 527 536 2485
WEIGHT (%) NUMBER (%) SPECIE PREY GROUP
2002 2003 2004 2005 2002 2003 2004 2005 Echinodermata 96.0 32.2 67.0 60.5 95.5 68.2 56.7 87.9 Other invertebrates 5.3 0.1 0.1 9.1 0.8 0.9
G. cynoglossus Pisces 1.4 5.0 5.9 0.2 0.3 0.3
Crustacea 15.3 3.7 15.3 10.2 32.5 21.7 38.6 32.7
Mollusca 2.1 0.4 0.5 1.0 8.4 3.3 3.9 6.9
Echinodermata 0.0 0.1 0.0 11.6 0.1 0.5 0.0 0.7 Other Invertebrates 80.6 93.2 79.1 70.8 58.2 72.3 56.8 57.0 Other/not identified 2.0 1.2 0.1 0.6 0.8 1.9 0.4 2.4
L. ferruginea Pisces 49.2 30.0 37.6 56.9 1.5 0.8 0.8 1.4
Crustacea 24.6 28.2 32.3 24.2 67.1 62.1 68.9 91.3
Mollusca 2.3 2.7 3.1 0.8 3.4 1.2 12.1 1.9
Echinodermata 1.8 1.2 1.6 0.7 0.8 0.6 0.6 0.2 Other invertebrates 20.3 37.2 24.1 16.3 27.1 35.1 17.4 5.0 Other/not identified 1.8 0.6 1.3 1.0 0.2 0.1 0.2 0.3
Analysis of the fish diet shows that invertebrates constitute an important diet component of some fish species, with most of the invertebrates fed upon being benthic. Benthic invertebrates reach 90% in weight and number in the stomach contents of some cases. Changes in the prey composition in wolffish diet have been observed within recent years. For example, the prey of A. denticulatus has shifted to a lower trophic level, with fish being the major prey in 2002, and molluscs and other invertebrates thereafter. Dominant prey in A. lupus has changed to molluscs and echinoderms, while A. minor became more piscivorous. It would be necessary to conduct a more extended quantitative analysis to establish the importance of these changes.
In the cases of species more specialized, or less opportunistic, detailed studies of the diet changes in the medium term may indicate changes in the ecosystem, particularly changes in the abundance of certain benthic organisms. On the contrary, a more constant diet in selective predators may indicate stability in the benthic ecosystem. The medium-term studies of the diet of witch, which seems quite stable, could be a good indicator to detect changes in the benthic habitat in the area NAFO.
In the life cycle of most fish species, the association with the benthic habitat is more marked in the first years of the life, when the diet is composed almost exclusively of benthic invertebrates. For this reason, the feeding studies focusing on the first years of life of demersal species can be very illustrative of the benthic association (and habitat), while in the adult stage, its relationship with the benthos become weaker.
Studies of feeding behaviour should not be limited to commercially important species but should also focus on species that give the highest level of information on benthic habitats and can best serve as indicators of changes in benthos communities and trophic relationships.