Fish form schools, layer or patches in which the individual fish's behaviour is not independent of
its neighbours movements. On the other hand, at low densities fish may have the freedom to act
as single individuals independently of what other fish are doing. Potentially, if these contrasts
occur in nature, they may give rise to behavioural differences of fish in front of the trawl at high
and low densities with successive effects on catchability and bottom trawl indices of stock
abundance.
We explore in this paper the hypothesis that the density of fish has a significant effect on
catchability of the survey trawl. Data from Norwegian and Canadian video observation recorded
during trawling are studied, and related to bag trawl experiments which measured escapement
underneath the survey trawls in both regions

Aglen, Asgeir

Engås, Arill

Godø, Olav Rune

McCallum, Barry R.

Stansbury, Don

Walsh, Stephen J.

Fish form schools, layer or patches in which the individual fish's behaviour is not independent of
its neighbours movements. On the other hand, at low densities fish may have the freedom to act
as single individuals independently of what other fish are doing. Potentially, if these contrasts
occur in nature, they may give rise to behavioural differences of fish in front of the trawl at high
and low densities with successive effects on catchability and bottom trawl indices of stock
abundance.
We explore in this paper the hypothesis that the density of fish has a significant effect on
catchability of the survey trawl. Data from Norwegian and Canadian video observation recorded
during trawling are studied, and related to bag trawl experiments which measured escapement
underneath the survey trawls in both regions

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Density dependent catchability in bottom trawl surveys

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Not to be cited without prior reference to the authors International Council for the Exploration of the Sea Density Dependent Catchability in Bottom Trawl Surveys by l ICESCM 19971W:T6 Theme Session W: The catching Performance of Fishing Gears Used in Surveys A. Aglen1, A.Engås1, O. R. Godø1, B. McCallum2 , D. Stansbuif, and S.J. Walsh20 1 Institute of Marine Research, P.O. Box 1870, N-5025 Bergen Norway 2 Northwest Atlantic Fisheries Center, P.O. Box 5667, St. John's Newfoundland Canada A1C 5X1 Abstract Fish form schools, layer or patches in which the individual fish's behaviour is not independent of its neighbours movements. On the other hand, at low densities fish may have the freedom to act as single individuals independent! y of what other fish are doing. Potentially, if these contrasts occur in nature, they may gi ve rise to behavioural differences of fish in front of the trawl at high and low densities with successive effects on catchability and bottom trawl indices of stock abundance. W e explore in this p aper the hypothesis that the density of fish has a significant effect on catchability of the survey trawl. Data from Norwegian and Canadian video observation recorded during trawling are studied, and related to bag trawl experiments which measuredescapement undemeath the survey trawls in both regions. Introduction Catchability is generally assumed constant between surveys, i.e. varying without trend, when estimating swept area indices from bottom trawl surveys. However, it is well recognised that this assumption is incorrect. Fish behaviour, density, maturity, light intensity, physiological condition, the environment, and traw l performance are some of the main factors that will cause catchability to vary. Such changes in catchability can create biases in the time series and reduce the precision of the abundance indices. For example Godø ( 1994, 1995) suggested that abundance of Barents Sea cod (Gadus morhua) indices would be negatively biased downward when stock density was low and positively biased upward when density was higher due to a strong link between density 0 Authorship in alphabetic order l and fish behaviour in the trawl path during the fish capture process. These changes in stock density were thought to be related to variation in the distribution pattem of the stock whereby fish may occur as single individuals scattered across their habitat or concentrated in patches. Recent studies show that fish escapement undemeath the survey trawl can be quantified by attaching bag trawls undemeath to capture downward escaping fish (Engås and Godø 1989, Walsh 1992, Godø and Walsh 1992; Dahm and Weinbeck 1992). We explore the hypothesis, which arose from qualitative video studies of cod, haddock and flatfish that fish behaviour in front of the trawl and hence escapement undemeath the trawl are affected by variation in fish density during the catching process. We will use data from bag trawl experiments conducted in the Barents Sea and off the coast of Newfoundland and discuss the significance of these finding in terms of reliability of survey trawl indices. Materials and Metbods The Norwegian data was collected of the northem Norway coast in 1995 (Table l) using the standard Norwegian survey trawl: the Campelen 1800 shrimp trawl. This three bridle shrimp trawl is rigged with 40 m sweeps, 35.6 cm diameter rockhopper footgear and a 20 mm mesh size codend. For the purpose of catching escaping fish three bag trawls were mounted under the trawl as described by Engås and Godø (1989). Tow duration was in most cases 20 minutes at about 3 knots. The Canadian data was collected off the east coast of Newfoundland in 1994 using two trawls: l) the old standard survey trawl, the Engell45 High Lift otter trawl rigged with three bridles 54 m sweeps, with 35.6 cm rockhopper footgear and a 29 mm codend liner; and 2) the new standard survey trawl, the Campelen 1800 shrimp trawl rigged with 40 m sweeps, 35.6 cm diameter rockhopper footgear and a 12 mm mesh liner in the codend ( see McCallum and Walsh 1996 for rigging details ofboth gears). Similar to the Norwegian experiment three bag trawls were mounted undemeath the main trawl covering 100% of the fishing area of the main trawl as illustrated in Walsh (1992). Tow duration for all experiments were standardised to l nm. The tow duration used a standard survey towing speed of 3.0 knots and a tow duration of 15 minutes. Table l Norwegian and Canadian ba~ trawl exEeriments COUNTRY YEAR MO NTH GEAR SPECIES NO. TOWS Canada 1994 January Campe len c od 23 plaice Canada 1994 January Engel c od 21 plaice Norway 1995 Campelen c od 45 haddock 2 Analysis From the Norwegian data, both cod and haddock (Melanogramus aeglefinus) were in sufficient numbers to be used in the analyses. Cod and American plaice (Hippoglossoides platessoides) were analysed from the Canadian experiments. The density of fish was determined from the catch data (sum of codend and bag catches). Schooling of fish normally occur by size. The catches were therefore allocated to three size groups which constituted the density information used in the analysis. The size groups were: Size Cod and Flatfish haddock Small 0-29 0-19 Medium 30-49 20-29 Large 50+ 30+ The hypothesis of density dependency of the under trawl efficiency is tested through an analysis of variance model (ANOVA). Efficiency is calculated as the trawl catch proportion of sum of trawl and bag catches. Escapement under the trawl is normally strongly fish size dependent. Therefore size together with density was used as explanatory variables and time (da y or night), and species were class variables in the model. In the Norwegian experiment tows were not conducted at dawn and dusk. In the Canadian experiments day was defined as the period when the sun was above the horizon. The analysis were done by SAS Lab (SAS 1992). A power transformation was necessary to avoid violation of assumptions done in the ANOV A. Results and Discussion Fish behaviour Based upon examination of several video recordings of fish in the mouth of the trawl we offer a qualitative description of fish behaviour for round fish and flatfish as follows: Codlhaddock We classified three predominant density related behaviours that was seen independently in both the Norwegian and Canadian experiments and was als o evident in other video recordings from other monitoring events (see Fig. 1). Loners : When l or 2 medium to large size cod are present in the recordings they were seen c lose to the front of the footgear and exhibited a characteristic kick and glide swimming motion behaviour across the trawl mouth. These fish tend to stay very close to the bottom and their turn-over rate is high. During these movements the cod are actively searching for gaps in footgear rigging or bottom contact to escape. 3 Schoolers : When 5 or more fish are present together in the recordings they form a school and exhibit uniformed behaviour as they swim orientated to the tow direction. They attempt to maintain greater distances between themselves and the footgear and exhibit very little frightened behaviour. Generally when one fish turns to enter the trawl several of the others will follow. The fish are approximately 0.5 to l m off bottom and their tumover and escapement rates are lower in comparison with the loners. A ... _1 .. ... ::;~ \ \ \ ' ' 4 6m '- .... ______ ... Figure l. Schematic presentation of distribution and behaviour in front of the ground gear of loners (A) from different situations (1-4), and loners-schoolers (B, schoolers are encircled). Loners-schoolers: At times loners and schoolers will appear in the video recordings together. In this case the loners do not join the schooling group but stay behind it and act independent! y in the manner described above. The charateristic crossing behaviour of the loners seems to be interupted in this situation. 4 Flatfish Flatfish ( doesn' t include the halibuts) appear to react independent! y with various components of the footgear, i.e. are non-schoolers. Individual flatfish stay very close to bottom in front of the footgear and movements are general! y laterally upon close approach of the footgear. After repeated encounters with footgear flatfish will either slow down their swimming (or stop) and allow the gear to pass over them or take advantages of gaps in the gear to escape under the footgear. In front of the footgear, the flounder's ability to manoeuvre becomes critical and appears to be a function of density of fish in the trawl mouth. As density increases, flatfish will tend to move a little higher off bottom and when tired will burst upward and turn into the net. Flatfishes arriving in the mouth area also encounter other flatfishes routed for the first time off the bottom by the footgear. Since behaviour reaction to a trawl component is generally in a lateral direction and not forward as in roundfish, swimming is often disrupted to prevent collisions with these other fish routed from bottom. When this happens, the fish lift off bottom, turn or flip backwards into the net and the probability of escape under the footgear is lowered. COD DIUR=NIGHT 10~SlZE 1519 Figure 2. Length (L) and density (NSIZE) dependency of efficiency on cod as observed in the Canadian experiment. Efficiency = codend catch/sum of catch in codend and bags. 5 Trawl experiments The statistical analysis show that trawl efficiency is a function of length and density in both the Canadian and the Norwegian data (Tables l and 2). The length effect is not surprising sinceEngås and Godø (1989) and Walsh (1992) showed that trawl efficiency was size dependent in gadoids and flatfish. Time of day and species effects are significant in Norwegian data but not Canadian data. Diel difference in escapement of cod may differ across geographical regions and within a region. W alsh ( 1991) show ed that escapement was significantly higher at night in flatfish but not c od. Figure 2 shows an example of variation of efficiency related to density and length. The density dependency is in many cases difficult to separate from other factors of importance. When data are run in the ANOVA model density always comes out as a significant factor, sometimes alene and other times in interaction terms with other factors. Observed and modelled efficiencies for the Norwegian experiment are compared in Figure 3. Using flatfish and roundfish in the same analysis as done in the Canadian experiments may confound the results. All studies of fish behaviour in the catching process show that these two gro up of fishes behave different! y (see behaviour part above) and in retrospect these groups should have been treated separately. However, it appears from the video studies that the interaction between individual within the groups is in both cases modifying the behaviour as density increase. Further, this change in behaviour, although for different reasons, reduces the number of escapes under the traw L + + co + 9= cf$ ~ * + ~ + 1 ~ 0.50 0.75 100 Ob5erved EFF**aa Figure 3. Observed- predicted plot based on the final ANOVA model for the Norwegian experiment. 6 W e recognise that there are some limitations in this present approach for analysing the density effect on survey trawl efficiency. Firstly, we are restricted to investigating the effect of fish density on escape rates in the trawl opening, i.e. the area covered by the trawl bags and cannot comment on the area between the wings and trawl doors. Secondly, using catch as the applied density measure may not necessarily be the best one. In theory, the same catch can be obtained from a standard haul when the study area is occupied by evenly distributed individuals at a low density compared to when hitting the only patch of fish in the area containing the same number of individuals. The physical configuration of fish prior to arrival of the vessel, i.e. under undisturbed conditions, is unknown. Video observations of the trawl mouth from experimental situations, like those presented here, indicate, however, that fish normally appear in the area in front of the footgear at varying rates. At increasing density, the fish seems to appear in front of the trawl in form of an increasing number patches. W e, therefore think that it is a fair assumption to u se catch as a reasonable proxy for density. Further, in the type of exploratory model used here, with several important variables entered, the output results were expected to vary substantially. Table l. Results from the ANOVA model run on the Norwegian data. More details are given in. Response: EFF3·2 . For the over all model R2 = 0.62. Source DF SS MS Species l 1.426 1.426 Length l 0.745 0.745 Length*Species l 0.331 0.331 Length*Density l 0.320 0.320 TIME l 0.108 0.108 Density l 0.103 0.103 Species*TIME l 0.0665 0.0665 Density*Species l 0.0519 0.0519 Density*TIME l 0.0177 0.0177 Length*TIME l 0.0052 0.0052 F 55.97 29.23 13.01 12.56 4.251 4.054 2.612 2.039 0.694 0.203 Pr>F 0.0000 0.0000 0.0003 0.0004 0.0398 0.0447 0.1068 0.1540 0.4053 0.6525 W e strongly want to emphasise that the premises for this paper was an investigative exercise to elucidate the problem. Nevertheless, we are encouraged that similar experiments from two geographical areas gave similar supporting results. Surveys are supposed to gi ve precise information on abundance and composition of stocks. If our hypothesis is valid, surveys might underestimate stocks when low and overestimate when high. For example, between 1983 and 1991, the average catch per tow of cod off the N ortheast coast of Newfoundland ranged from 80 to 205 fish per tow, from 1993 to 1996 it has dropped to around 2 cod per tow. In this critical situation, if o ur hypothesis is correct then o ur understanding and ability to evaluate stock development is potentially low. Recent discussion and simulations in the ICES Arctic Fisheries WG on the application of different catchability profiles in the tuning of the VP A of Northeast Arctic cod have underlined the need for hetter understanding of which factors are important in contributing to variation in catchability (ICES Arctic Fisheries W orking Gro up, August 1997). 7 Conclusions This p aper supports the initial hypothesis of dens i ty dependence effect on catchability. W e recognise the limitations of the data but believe we have demonstrated that there is a possible connection between density and catchability. W e recommend that further studies which improve o ur understanding of natural and trawl induced behaviour along with studies of the interaction behaviour in demersal species are strongly needed to test this hypothesis. Table 2. Results from the ANOV A model run on the Canadian data. Response: EFF . For the over all model R2 = 0.33. Source DF ss MS F Pr>F Density l 0.492 0.492 9.912 0.0021 Length *Spee i es l 0.456 0.456 9.176 0.0031 Length l 0.333 0.333 6.702 0.0109 Density*Species l 0.317 0.317 6.384 0.0129 Time l 0.193 0.193 3.892 0.0510 Time*Species l 0.152 0.152 3.067 0.0827 Spee i es l 0.0974 0.0974 1.962 0.1641 Length *Time l 0.0562 0.0562 1.133 0.2896 Density*Time l 0.0374 0.0374 0.753 0.3873 Length *Density l 0.0057 0.0057 0.114 0.7358 References Dahm, and H. Wienbeck 1992. Escapement of fish undemeath the groundrope of a standard bottom trawl used for stock assessment purposes in the North Sea. ICES CM 1992/B:20 Engås, A. and O.R. Godø 1989. Escape offish under the Norwegian sampling trawl and its influence on survey results. J. Cons. int. Explor. Mer. 45: 269-276. Godø O.R. and S.J. Walsh 1992. Escapement offish during bottom trawl sampling- implications for resource assessment. Fish. Res., 13:281-292. Godø, O.R. 1994. Factors affecting reliability of groundfish abundance estimates from bottom trawl surveys. In A. Ferno and S. Olsen [eds.] pp 166-199. Marine Fish Behaviour in Capture and Abundance Estimation. Fishing News Books. Oxford. McCallum and Walsh 1996. Groundfish survey trawls used at the Northwest Atlantic Fisheries Centre, 1971-present, NAFO Sei. Coun. Studies, 29:93-104 Walsh S. J. 1992. Size dependent selection at the footgear of a groundfish survey trawl. N. J. Fish. Mgmt. 12:625-633. Walsh S.J. 1991. Diel variation in availability and vulnerably of fish to a survey trawl. J. Appl. Ichthyol.7: 147-159. 8 

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Density dependent catchability in bottom trawl surveys

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