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Not to be cited without prior reference to the authors

International Council for CM 1997/W:13 the Exploration ofthe Sea Theme Session: \V The Catching Performance of Fishing Gears used in Surveys

PRELIMINARY ANALYSIS OF THE SWIMMING ENDURANCE OF ATLANTIC COD (GADUS MORHUA) AND AMERICAN (HIPPOGLOSSOIDES PLATESSOIDES)

• by

Paul D. Wingert, Pingguo Hel, and Stephen J. Walsh2

IFisheries & Marine Institute, Memorial University ofNewfoundland, P.O. Box 4920, S1. JoOO's, Newfoundland, AIC 5R3

2Department ofFisheries & Oceans, Science Branch, P.O. Box 5667, S1. JoOO's, Newfoundland, AIC 5Xl

ABSTRACT

Swimming flume studies were eonducted for Atlantie eod and Ameriean plaiee to investigate the effects ofwater temperature and fish size on swimming endurance. Fish were tested on a routine basis over a duration of29 weeks from the fall of 1996 to the spring of 1997. Swimming trials far cod were tested aeross a range offish sizes (41 to 86 em), temperatures (0.0 to 9.8 °C), and swimming speeds (0.6 to.l.3 m/s). Swimming trials for plaiee were tested across a range offish sizes (14 to 44 em) and temperatures (-0.2 to 9.6 °C) at a swimming speed ofO.3 m/s. A preliminary examination ofthe data was eondueted using multiple linear regression. The results do not support the hypothesis oftemperature related bias in catching performance for either species. In cod, swimming speed was the only significant factor affecting swimming endurance. Fish length largely influeneed swimming endurance in plaice, hut had no apparent effect on swimming endurance in eod.

1 INTRonUCTION

Identifying and measuring the factors that affect survey trawl catchability has been the subject ofmuch research in recent years (see for example, Godo & Walsh 1992; Dickson 1993a, 1993b; Godo 1994; Walsh 1996; Somerton & Munro 1996). Changes in environmental conditions in the survey area arid biological constraints within the targeted species are two sources ofbias which can increase variance around abundance indices. Several variables ofthis nature are known to affect trawl-induced herding and avoidance behaviours, leading to variation in catching efficiency. Chiefamong these include ambient light intensity and fish size (Parrish et al. 1964; Wardie 1983, 1986; Walsh 1991, Walsh & Hickey 1993). Other factors may include age, physiological condition, arid bottom temperature (Laevastu & Favorite 1988; He 1991, 1993). With the possible exception oflight intensity, each is thought to affect swimming endurance in the trawl path in some manner. Unfortunately there is little empirical data to support these theories.

Fish size and water temperature are generally thought to be two ofthe most important variables affecting prolonged swimming performance (Le., endurance). Direct video observation near the bosom ofbottom trawls has " demonstrated the scale-effect in trawl-induced swimming endurance (Main & Sangster 1981). It is weIl established that a typical fish can swim forward 0.7 body lengths for each complete tailbeat cycle (WardIe & Videler 1980). Consequently, smaller fish can be seen to swim more vigorously than larger fish in order to maintain position in the • bosom ofa trawl, resulting in an earlier onset offatigue (Wardie 1983, 1986). The effect ofbottom temperature on trawl-induced swimming endurance, however, is not as apparent. Video observation ofthe capture ofwalleye pollock (Theragra chalcogramma) by Inoue et al. (1993) indicated a very weak swimming response by fish at low temperatures. In contrast, Walsh & Hickey (unpublished) have witnessed Atlantic cod (Gadus morhua) swimming in the trawl mouth with ease for considerable time and distance at temperatures weIl below 0 °C.

A limited number oflaboratory studies have measured the swimming performance ofcommercial groundfish species. Among these, there has been no systematic attempt to measure swimming endurance over a range of temperatures and fish sizes for either AtIantic cod (Gadus morhua) or American plaice (llippoglossoides platessoides). In the case ofcod (Beamish 1966; He 1991; Nelson et al 1994), the result has been a compilation of experiments in which the swimming apparatus, training routine, and temperature treatments differed markedly. Similar studies with species have also been conducted (Beamish 1966; Priede & Holliday 1980; Duthie 1982), but with no investigation into the swimming performance ofAmerican plaice. This paper will present a prelirriinary analysis ofa systematic investigation ofswimming endurance in both these species across a range of temperatures and fish sizes using a standardized methodology. The results are discussed in relation to other swimming studies and the implications for research surveys.

MATERIALS & METHOnS

Swimming endurance was investigated in Atlantic cod and American plaice across a range ofwater temperatures and fish sizes using a swimming flume tank (He 1991). In addition, cod were tested across a range ofswimming speeds. Trials were completed on a routine basis over a duration of29 weeks from the fall of 1996 to the spring of 1997. A total of229 cod swimming trials, and 228 plaice swimming trials were completed during the study.

Apparatus:

The swimming flume is a 1/8 scale model ofthe flume tank located at the Fisheries and Marine Institute, and has a maximum working section of 1.8 X 0.5 X 0.46 meters (IengthXwidthXdepth). The total volume ofthe flume tank is 3450 litres. Three electric 1 HP pumps deliver a continuous flow, variable between zero and 1.76 rnIs. The floor of the flume is also equipped with a variable speed belt designed to simulate the passing ofground (i.e., in the wild) as the fish swims. This moving belt eliminates boundary layer effects near the bottom and enhances uniformity ofthe flow. Perhaps the most functional use ofthe belt is to prevent the flatfish from adhering to the bottom ofthe flurne,

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forcing them to swim continuously. Tests were conducted at the Ocean Sciences Centre ofMemorial University of Newfoundland at Logy Bay, Newfoundland.

FisII:

Cod and plaice were captured from a variety oflocations throughout the year to support the experiment (Table I). Tbe fish were transported to the Ocean Sciences Centre for tank adaptation and endurance testing. A minimum tank adaptation oftwo weeks was required for all specirTIens before endurance testing. Cod were kept in a large raceway compartment (2.5x2.5xl.O m) while the plaiee were kept in a smaller 2.0x2.0xO.5 m fiberglass tank. Both tanks received a continuous supply ofambient temperature seawater. Fish were fed on a maintenance diet ofchopped Atlantie herring (Clupea harengus) onee a week. A food ration ofapproximately 2% (winter/spring) and 5% (summer/fall) ofthe mean body weight per week was maintained. None ofthe fish were tested within 48 hours following feeding. Only fish that appeared to be in good condition (i.e., good colour and weight) were used.

Treatments:

Tbe flume Was operated on a flow-through basis with a continuous supply offresh ambient temperature seawater • from the nearby Logy Bay. Seasonal changes in temperature during the 29-week study provided a range of temperatUres (-0.2 - 9.8 oe) for testing swimming endurance. Natural short-term fluctuations in the ambient temperature were ignored and no attempt was made to artificially manipulate the temperature from ambient. Tbis technique allowed us to test the swimming endurance offish across a range oftemperatures, changing slowly with natural seasonal change.

Fish size was treated as a continuous variable, and every attempt was made to test fish across a broad length range. Cod ranged in length from 0.41 to 0.86 m with a mean length ofO.58 m, while plaiee ranged in length from 0.14 to 0.44 m with a mean length of0.31 m.

Swimming speed treatments were chosen based on initial pilot trials during the laie summer of 1996. Similar to He (1991), cod were found to swim comfortably up to a maximum prolonged speed (Urop) of 1.3 mls in the flume tank. Speed treatments for this species were randomly assigned between 0.8 and 1.3 mls, at increments of0.1 mls. A number ofadditional trials were also conducted at 0.6 mls at temperatures below 3°C. Plaice on the other hand were more difficult to condition for steady swimming behaviOllr within the flume tank. Our initial pilot trials resulted in almost 100% faUure at conditioning plaiee to swim at speeds greater than 0.5 mls. Due to this low

" percentage ofsuccess, a single speed treatment of0.3 mls for plaiee was ultimately chosen for the experimental design. A calibration offlume speeds was conducted using a Seba mini-current meter (Geneq Ine., Model 486). • JUetllOdolog)': A number ofpilot swimming trials were initially conducted for both species to determine whieh experimental conditions would best encourage natural swimming behaviour within the flume tank. Among these were water depth, downstream grid voltage, and the ambient light intensity. Cod were found to swim best at the maximum water depth of0.46 m. However, when tested at this same depth, the plaieeoften 'flared-up' perpendieular to the swimming plane becoming considerably disoriented. Tbey also tended to display the recognizable behaviour of many flatfish species in captivity by occasionally swimming near the water surface. Tbe vertical nature ofboth these swimming behaviours was not conducive for swimming into a current, often leading to cases ofhigh drag and an immediate fall-back to the electrified grid. To curb these behaviours and encourage forward s\vimming irito the current, the water depth was redueed to 0.26 m, and a floating Iid apparatus was introduced into the flurne. Tbis modification prevented the plaice from swimming near the surface and reduced the Iikelihood of'flare-up' behaviour leading to high drag.

Swimming was encouraged by placing pairs ofelectrodes in the downstream end ofthe swimming flurne. A

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pulsing stimulus (2 Hz) with peak voltage ofapproximately 15 V (cod) and 8 V (plaice) was applied to encourage the fish to swim against the flow until exhaustion. Prodding techniques used in similar studies (Beamish 1966; Taylor & McPhaill985) were not used in order to avoid any subjectivity introduced by the experimenter. Dur initial pilot trials also suggested an increased sensitivity to a pulsing stimulus rather than a constant voltage stimulus used in other studies (Beamish 1966; He 1991). Dur choice ofa comparatively high voltage was based on our intention to produce a moderate 'fear response' similar to the stimuli produced by approaching bottom trawl components such as the doors, bridles, and footgear.

Swimming trials for both species were performed under conditions ofsubdued lighting. Consistent with the methods ofBe (1991), an area ofbright light (1.44 x 101 LUX) was projected near the downstream end to encourage cod to swim away from the downstream electrical grid. This technique was less effective in conditioning plaice and was therefore not used.

Cod were required to perform a standardized training routine before beginning their endurance testing. Tbis routine began with a minimum 10 minute acclimation period within the flume with a nominal flow of0.1 mls. Tbis was followed by a 10 minute orientational swim at Y2 body Iengths per second (BUs) (Vo)' Tbe flume belt, downstream lighting, and electrical grid were activated at the beginning ofthe orientational swim. If the prescribed speed (VJ was less than 1.0 BUs, the swimming speed was gradually increased from Vo to VI over aperiod five minutes. If however VI was greater than 1.0 BUs, then the orientational swim was followed by an extra conditional swim at 1.0 BUs (VJ for an additional five minutes. Following a conditional swim, the swimming speed was gradually increased from Vc to VI over the next five minutes.

Plaice were tested using another standardized routine. Similar to the cod, this routine began with a minimum 10 minute acclimation period within the flume with a nominal flow of0.1 mls. Tbe flume belt and downstream electrical grid were then activated, and the swimming speed was gradually increased to the prescribed speed (VJ of 0.3 mls. Tbe time taken to reach VI was dependent on the apparent comfort ofthe fish, typically ranging between three and eight minutes. Tbe flexibility ofthis routine was necessary given the often unsteady and unpredictable nature ofplaice swimming behaviour.

Swimming endurance for both species was defined as the period oftime the fish was able to swim at the prescribed speed before leading to exhaustion. Exhaustion was noted when the fish was no Ionger able to lift offthe downstream netting for a dumtion of 10 seconds. In cases where endurance exceeded 200 minutes, the trial was terminated and the experimental conditions were assumed to be within the aerobic scope ofthe fish.

Experimental Replication: Given the limited numbers offish and tank space available, testing the same fish a number oftimes was necessary • for suitable replication. Experimental fish were selected from the holding tanks haphazardly without replacement in order to optimize the recovery period before retesting any given fish again. Tbe minimum recovery period was generally one week. With the frequent arrival ofnew fish (Table 1), it was estimated that the maximum number of trials for any given fish did not exceed four.

Analysis:

A preliminary examination ofthe data was conducted using multiple linear regression. Tbis technique was chosen based on our interest in determining the best mathematical models for describing the relationship between swimming endurance and several continuous independent variables, namely temperature, fish length, and swimming speed. Dur goal was to determine the most parsimonious model for each species. Tbis meant that the 'best' models were taken to be those in which all variables were significant (a = 0.05), and each variable contributed to the significant reduction (a = 0.05) ofthe sum ofsquared errors (SSE). Variable selection was undertaken using an iterative stepwise methodology.

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, . , We excluded trials on the basis ofa) the fish did not reach the prescribed swimming speed (VI)' b) did not show natural swimming behaviours, and c) where swimming endurance exceeded 200 minutes. Tbis yielded a total of 139 trials for cod and 84 trials for plaice to be used in the regression analysis. Normality ofthe datasets was assessed by examination ofthe histograms, normal probability plots, and residual plots. Certain data transformations were necessary in order to satisfy the assumptions ofnormality and homogeneity ofvariance. Logarithmic transformation (loglo) offish length scores was beneficial for cod, but was not necessary for plaice. Measures ofswimming endurance were loglo transformed for both species. Temperature had an approximately uniform distribution, except for a nominal loss ofscores betv.een 3 and 4°C for both species. Tbe following general linear model was chosen to linearise endurance values for cod;

log E = Po + Pss' Xss + Pn . (Ioglo xn ) + PI . XI + 4 interaction terms

and for plaice;

log E = Po + Pn . Xn + PI . XI + 1 interaction term

where E is the predicted value for swimming endurance in minutes, Xss is swimming speed in cm/s, Xn is fish length in cm, and XI is water temperature in °C.

RESULTS

Atlantic Cod:

Swimming speed was the only significaD.t factor affecting the swimming endurance in Atlantic cod (r2 = 0.674, P < 0.0005). Tbe logarithm ofendurance decreased linearly with increasing swimming speed (Fig. 1). Tbe mean swimming endurance at 0.6 rn/s was 50 minutes, with the endurance ofsome individuals falling to less 1 minute at speeds exceeding 1.1 m/s. At lower speeds the dominant s\vimming strategy was typically cruising, while a combination ofcruising and burst swimming was always observed at higher speeds.

Cod swimming endurance was not significantly affected by water temperature (p > O. I). Figure 2 illustrates the studentized residuals (Le., standardizing endurance by removing the dominant trend due to swimming speed) plotted against water temperature. Tbe figure shows little remaining trend due to water temperature. For comparison, Figure 3 shows the temperature range grouped into four bins. Tbe slopes and intercepts ofthese trajectories can be seen to be very similar, indicating a strong lack ofthermal dependence. No interaction was detected between water • temperature and swimming speed. Fish length had no apparent effect on cod swimming endurance (p > 0.1). Figure 4 illustrates the studentized residuals (Le., standardizing endurance by removing the dominant trend due to swimming speed) plotted against the logarithm offish length. Tbe figure shows little remaining trend due to fish length. Swimming speed in centimeters per second (crn/s), therefore, proved to be the better predictor ofswimming endurance (r2 = 0~674) than did the specific swimming speed expressed in body lengths per second (BLls) (r2 = 0.308). Despite the insignificant main effect, we did fmd evidence ofan interaction (in one model) between fish length and swimming speed (p < 0.05). Tbis was determined to be an artifact ofmulticollinearity, and the teim was subsequently removed from further models. General observation indicated that the choice and frequency ofswimming strategy (e.g., cruising vs. burst­ and-coast) may have been a function offish length. Variation in swimming strategies between fish was not taken into consideration for this preliminary analysis.

Tbe maximum sustained swimming speed (Ums) could not be resolved within the range ofspeeds tested. Despite this, many trials were recorded at speeds 0.6 and 0.8 m/s in which swimming endurance exceeded 200 minutes (N=15 trials). Tbe mean fish length for these trials was 0.56 in (s.d. = 0.085), occurring only at the lower end ofthe

5 temperature range (0.1 - 2.7°C). Pereent dissolved oxygen saturation forthese trials (x = 109.17%, s.d. = 7.67) was only slightly higher than reeorded for those trials where swimming enduranee was less than 200 minutes (x = 102.80%, s.d. = 8.52).

The least-squares linear regression equation for swimming enduranee in eod was redueed to only a single predietor variable, namely swimming speed. Terms for temperature, fish length, and their associated interaetion terms were removed based on low levels ofsignifieanee (p> 0.5) or diffieulties with multieollinearity. The resulting equation is;

log E = 3.096 - 0.023' Xss (r=0.674) where E is the predieted value for swimming enduranee in minutes, and x.. is swimming speed in em/s.

American plaice:

The swimming enduranee ofplaice was largely dependent on fish length (p < 0.0005) (Fig. 5). The mean • swimming enduranee for 0.20 to 0.25 m fish was 3.3 minutes, rising to almost 51.1 minutes for 0.35 to 0.40 m fish (2-3°C). Smaller fish « 0.25 m) often struggled, opting for a burst-and-settle swimming strategy with exhaustion shortly following. Larger fish by eomparison typieally assumed a steady eruising swimming strategy, opting for a burst-and-settle behaviour only near the onset ofexhaustion. The maximum specific swimming speed reeorded was 1.76 BUs. While the pereentage ofsueeess for this species to reaeh the preseribed swimming speed was low (42%), it did not appear to be a funetion offish length. Trials in whieh swimming enduranee exeeeded 200 minutes (N=I0) were observed aeross most temperatures (0.3 - 9.7°C), but only among larger individuals (x = 0.36 m, s.d. = 0.05 m). Pereent dissolved oxygen saturation for these trials (x = 91.93%, s.d. = 5.57) was eomparable to those trials where swimming enduranee was less than 200 minutes (x = 92.72%, s.d. = 5.39).

Water temperature was found to have a weak effeet on the swimming enduranee ofplaice. Figure 6 illustrates the studentized residuals (Le., standardizing enduranee by removing the dominant trend due to fish length) plotted against water temperature. The figure shows only a weak remaining trend due to water temperature. When temperature was entered into the model as a lone main-effeet, the resulting level ofsignifieanee was poor (p > 0.1). However, when entered into the model along with fish length, the level ofsignifieanee for temperature was observed to drop substantially (p < 0.05). While the resulting signifieanee level may have warranted inclusion in the model, its eontribution in reducing the SSE for the model was negligible (F-droPll. BI) = 1.55, p> 0.1). On the basis ofthe eriteria outlined under Analysis, the term for temperature was removed from the model. The interaetion term for water temperature and fish length was highly eorrelated with fish length. This lead to symptoms of • multieollinearity, forcing the exclusion ofthe interaetion term from the model as weIl.

The least-squares linear regression equation was redueed to only a single variable, namely fish length, for predieting swimming enduranee in Ameriean plaiee. The temperature term and interaetion term (fish length x temperature) were withheld based on low reduetion ofthe SSE and multieollinearity respeetively. The resulting equation is;

log E = -0.822 + 0.062 • Xn (r= 0.429) where E is the predicted value for swimming enduranee in minutes, Xn is fish length in cm, and Xl is water temperature in oe.

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DISCUSSION

Temperature:

The effects ofwater temperature on the swunming endurance ofAtlantic cod may be more subtle than previously thought. Until now, the only available data had been collected by He (1991) at O°C and Bearnish (1966) at 5°C and 8°C. A comparison ofthese two datasets by He (1991) had indicated the likelihood ofsignificant tempeniture effects on swiminirig endurance at speeds ~ I m/s. However, Videler (1993) pointed out that it is difficult to objectively compare endurance data across studies due to the large variations in experimental eonditions. The swimming flume t3nk used in this study is the same flume tank used by He (1991). The working seetion ofthis flume tank is almost three times the sizeofthe Blazka-type respirometer used by Beamish (1966). Moreover, the eod used hy He (1991) were native to the relatively cold Newfoundhmd waters, \vhile those used by Bearnish (1966) were collected from the relatively warm waters ofthe Bay ofFundy. Nevertbeless, the eomparison ofthe two datasets provided by He (1991) was consistent with theb expected trend, namely a proportional decrease in s\vimming endurance across ariiunber ofswiriiming speeds, presumably due to a reduction in \vater temperature. Beamish's (1966) results indicated that cod (0.35 - 0.36 m) at 8°C could swim for about 16 minutes at 1.05 nils, compared to oitly 6 minutes at 5°C at the same speed. He's (1991) results showed that eod ofsimilar size (0.36­ 0.42 m) at O~C could only swun for about 2 minutes at I InIs. By fitting Beamish's (1996) data to his model at I m/s, He (1991) extrapolated that cod would have a swimming endurance of50 minutes at 5°C, and approximately 100 ininutes at 8°C. The firidings from the current study found no evidence ofthemial dependenee to support the suggested temperature effect in He's (1991) comparative approach. The endurance measures recorded here appear to fall intermittent to those published by He (1991) arid Beamish (1966), and ;'"ell belo\v the extrapolated values provided by He(1991). Due to the lack ofsize dependenee observed, pooled means are used for coinparison. This study revealed a mean endurance ofroughly 8 minutes at O°C, 6 minutes at 5°C, and 6 minutes at 8°C for trials similarlyat I InIs. Given that these endurance times at 5°C and 8°C do not nearly approach He's (1991) suggested times, it is possible that He's (1991) extrapolation ofBeamish's (1966) data was unrealistically high. In addition, supporting evidence suggests that the effects ofteinperature may be reduced at higher speeds. Neither Beamish (1966) nor the work published by Blaxter and Dickson (1959) on Baltic eod (Gadus morhua callarias) were able to derrionstrate thermal dependence at higher speeds (> I m/s). These fmdings are consistent \vith the current study in this manner.

For flatfish, there 1s little consensus in the literature on the swimmirig perfomiance related to temperature change. Much ofthe work has concentrated on measures ofcritical swimming speeds (UcriJ \vith Iittle attention to measures ofswirriming endurance. An analysis by Priede and Holliday (1980) observed an inereasing trend iri Ucrit values with increasing temperature for North Sea plaice (Pleuronectes platessa). They noted Ucrit values of 1.18 BLis at 5°C, 1.36 BLis at 10°C, arid 1.53 BLis at 15°C (0.29 - 0.33 in fish). Teinperature dependence ofUcrit values has also • been demonstrated in the Europem (Platichthysflesus) by Duthie (1982). In contrast, this same author detected no effect oftemperature on Ucrit values for the common dab (Limanda /imanda), or the lemon ( kitt). Critical swimming speeds observed by Duthie were 1.50 BLis for the , 1.30 BLis for the common dab, and 1.10 BLis for the lemOli sole. In terms ofswimming endurance, the only known study until now was Beamish's (1966) work with the (Pseudopleur011ectes americanus). While the author did observe incn~ased levels ofendurance at 14°C, hefoUnd no consistent effect oftemperatUre on s\vimming endurance across the cooler 5°, 8°, arid 11°C temperature treatments (0.19 - 0.23 m fish). Therefore, except for the European flounder and North Sea plaice, the relative thermal indeperidence reported in the cominon dab, lemon sole, and winter flounder is consistent with the undetected effects oftempcrature on swimming endurance for Amedcan plaice found in this study.

Principles ofmuscle kinetics suggest that temperatüre should largely affect the swimmirig endurance ofthe whole organism. For instance, the contractile properties oflateral swimming muscle (e.g., maximum velocity of shortening -Vmax) have been shown to be strongly influenced by temperature in both Atlantic cod (Videler & Wardie 1991) and at least two species offlatfish: the leinon sole and North Sea plaice (Wardie 1980). Rome (1990)

7 outlined that due to decreasing Vmu with decreasing temperature, cold muscle is unable to generate the same force and mechanical power as warm muscle. The author argues that since the power output ofaerobic muscle fibers at low temperatures is drastically reduced, additional anaerobic fibers must be recruited at a lower swimming speed ('compression ofthe recruitment order theory'). In other words, at a given speed within the range ofprolonged swimming speeds for a species, more anaerobic fibres will be recruited at a colder temperature in order to eompensate for the loss ofpower. The result should be an earlier onsel offatigue at lower speeds for lower temperatures due to a more rapid accumulation oflactic acid and/or depletion ofglycogen reserves.

Cold acclimation may account in part forthe undetected effeets oftemperature on swimming endurance in this study. It is suggested that thermal eompensation at eolder temperatures inay have reduced the apparent effect of temperature on swimming endurance. In Newfoundland waters, cod are typically distributed across a temperature range ofO.7 - 4.2°C (Lear 1984), arid have been observed overwintering in the inshore waters ofTririity Bay, Newfoundland at temperatures below -1.5°C (Wroblewski et aI. 1994). Plaice are inöst eommonly found at temperatures ranging from -0.5 - I.O°C (Scott & Scott 1988), but have been reported at temperatures as low as -1.8°C (Pitt 1975). The extension ofthese species temperature tolerance into sub-zero conditions may involve some aspect ofdensity-dependent temperature seleetion, as suggested by Swain & Kramer (1995) for eod in the Gulfof • St. Lawrence. Owing to the adaptive strategies (e.g., antifreeze substances) already known to exist in cod (Goddard & Fleteher 1994) and related flatfish species (Scott et al. 1987; Fletcher et aI. 1989)for survival at cold temperatures, it is suggested that other forms ofthermal compensation may exist. Guderley and Blier (1988) argue that in certain species this leads to physiological compensatory responses leading to an increase in the eapacity for sustained swimming at low temperatures. The authors outline several types ofpositive thermal eompensation, including changes in muscle contractile properties, myosin ATPase activity, the proportion ofred muscle fibers, and the levels ofaerobic enzymes in the musculature. Whether such thermal responses occur within Atlantic cod or American plaice has not been established. Since the fish used in this study were kept at ambient temperature, they therefore underwent a very slow and gradual change in temperature consistent with natural seasonal fluctuation. Vnder these conditions, we feel it is safe to assume that the fish were given sufficient time to acclimate to the changes in temperature, and ifcapable, the potential to develop thermal compensatory responses.

Fish Size:

Several factors may account for the undetected effect offish size on the swimming endurance ofAtlantic eod. It is suspected that between-individual variation in endurance may exist due to differences in swimming strategy. The large working section ofthe flume tank provided the fish the opportunity to employ both cruising and burst-and­ coast swimming strategies. These differences were not taken into consideration for this preliminary analysis. Post- analysis ofthe video footage may help to explain additional variation in the endurance measures. It is also possible • that the conditioning routine used (outlined under Methodology) may have disadvantaged the larger fish due to prolonged swimming at higher specific speeds. Further analysis ofthe data may require weighting ofthese particular swimming trials.

The fish size I endurance relationship for American plaice found in this study is comparable to the fmdings by Beamish (1966) for winter flounder. Both studies have reported increasing swimming endurance with increasing fish size over approximately the same size ranges. However, it is difficult to compare the datasets quantitatively, due largely to the signifieantly higher swimming speeds at which Beamish had to test his fish (0.75 - 1.35 mls). Vsing a horizontal B1azka-type respirometer, Beamish (1966) was unable to induce the fish to swim at speeds less than 0.75 mls (Le., the fish would otherwise adhere to the bottom). This problem was later overcome by Priede and Holliday (1980) and Duthie (1982) by usmg a tilting Brett-type respirometer. The distinct advantage ofthe moving floor belt used in this study is that the fish do not have to be foreed into swimming by generating high speeds, or by swimming downhill. Nevertheless, the maximum observed swimming speed in this study (1.76 BLls) was more agreeable with the Verit values reported by Priede and Holliday (1980) and Duthie (1982) discussed above. The relatively low eoefficient ofdetermination (r = 0.429) found in this study is thought to be a eonsequence ofthe inherent difficulty we faced in successfully conditioning American plaice to swim under flume conditions. Similar

8 .' t

to eod, it is suspeeted that between-individual variation iri endurance may exist due to differences in swimming strategy. The large workirig section ofthe flume tank provided the fish the opportunity to employ both eruising and burst-and-settle swimming strategies. These differenceswere not taken into eonsideration for this preliminary analysis. Post-analysis ofthe video footage may help to explain additional variation in the endurance measures.

Implications for CatchabiIity:

Variation in the eatchability ofcod due to reduced swimming capability at lo\\'er temperatures has been suggested byHe(199l) and Smith and Page (1996). Our results indicate that this suggestion may be incorrect. The lack of temperature dependence observed across the wide range oftemperatures, fish sizes, and s\vimming speeds tested in this study is evidence that temperature-related biases in trawl selectivity may be minimal. These findings are j>ärticularly relevant with respeet to herding, where swimming speeds during this part ofthe eapture process are typically less than 34% ofthe towing speed (deterniined from Engas 1994), and therefore weIl within the range of speeds used in this study. The lack oftemperature trend detected for the range ofswimming speeds tested (0.6 - 1.3 InIs) is suggestive that thermal dependence at speeds approaching survey towing speeds (e.g., 1.5 mls) is unlikely. Swain arid Nielsen (1996) eonducted an analysis ofeod survey data collected from 1971to 1991 in the southem GulfofS1. Lllwrence. The authors provided supporting evidence, eoncluding that variation in catchability was due to changes iri availability, arid eould not be attributed to cold water induced reduction in swimming performance.

Selectivity is expeeted to be strong durmg the herding process for flatfish species such as American platce (Waish 1992). Based on the endurance I fish length relationship defined above, this indicates that small changes in size selectivity could potentially translate into large differences in swimming endurance. Several variables may influence endurance (and therefore herding efficiency) offlatfish species, including towing speed, bridle angle, point ofinitial contact with the bridle, and the degree to which the bridle is in eontact with the sea floor (Main & Sangster 1981; Wardie 1983; Engas 1994; Somerton & Munro 1996; Walsh 1996). Based on direct video observation ofthe herding behaviours ofNorth Sea plaice, common dab, and lernon sole (0.19 - 0.34 m), Main & Sangster (1981) developed a generalized herding model for flatfish species. The authors described the avoidanee response as a burst-and-settle swimming strategy iri the direction 90° to the bridle's apparent line ofadvance. In order to avoid being overtaken by the bridle, the fish must swim at a speed equal or greater than the herding speed (UII)' This is afunction ofthe bridle angle and the forward towing speed ofthe trawl (Ur). Since bridle angles are normally iri the range of 12° to 20° (Engäs 1994), this eorresponds to UII values typically betWeen 0.3 and 0.5 mls. The swimming speed used in this study, 0.3 mls, represents the lower end ofthis range. However, even with thorough survey trawl standardization (McCaIlum & Walsh 1995), bridle angles are often highly variable between fishing sets. Annual groundfish surveys conducted by the Nofthwest Atlantie Fisheries Centre (NAFC), have determined that variability in bridle angle for the Campelen 1800 shrimp trawl (FRV W;lfred Templeman) can vary as much as 15° (7.4° to 22.6°, x = 19.2°, CV =15%; 1995 sUrVey data) between fishirig sets (Walsh & McCaIlum • 1997). This is equivalent to arange iri UII from 0.19 to 0.58 nils (Ur = 1.5 mls). Based on the endurance I fish length relationship defmed above, variation in Uil ofthis magnitude could potentially bias size seleetion pressures for American plaice. This should result in variability iri the herding efficieney ofsmall fish due to liniiting swimming capabilitY, resulting in differences in size eomposition ofthe eatches.

Swimming behaviour ofplaice in the flume tank was eonsistent with Main & Sangster's (1981) flatfish herding model. Plaice often used a burst-and-settle swimming strotegy, followed by riding back on the belt and a subsequent startle by the eleetrical grid. This process would often repeat itself for some time, and is analogous to the conseeutive disturbances during the herding by trawl bridles. It should be noted that rriany ofthe larger plaice \vere obserVed to use a cruising swimming behaviour. Whether this strategy was less energetieally costly (in terms oferiduranee) in comparison to the burst-arid-settle strategy has not been determined.

9 ..,

CONCLUSIONS

This study marks the first systematic attempt to investigate the effects ofwater temperature and fish length on the swimming endurance ofAtlantic cod and American plaice. The results from our preliminary analysis ofthe data do not support the hypothesis oftemperature related bias in catching performance for either species. It is suggested that thermal compensation may account, in part, for the lack ofthermal dependence detected. Fish length largely influenced swimming endurance in plaice, but had no apparent effect on swimming endurance in cod. Post-analysis ofthe video footage wiII be conducted in an attempt to explain additional variation in swimming endurance due to differences in swimming strategy.

ACKNOWLEDGEMENTS

This project was funded through Department ofFisheries and Oceans (DFO) Strategic Science Fund Project # 90040. Special thanks to Ross Wilson, Jim Devereaux, and the many members ofthe Ocean Sciences Centre's • facilities management stafffor their continued support during the study. George Legge is recognized for his electrical and mechanical assistance with the operation and maintenance ofthe swimming flume tank. Additional appreciation is also given to many individuals within the DFO Groundfish Division, including Dave Orr, Mick Veitch and Paul Higdon for their valued help with feeding and transportation oflive fish, and to Noel Cadigan for statistical consultation.

REFERENCES

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Dickson, W. 1993b. Estimation ofthe capture efficiency oftrawl gear. 11. Testing a theoretical model. Fish. Res., 16:255-272. • Duthie, G.G. 1982. The respiratory metabolism oftemperature-adapted flatfish at rest and during swimming activity and the use ofanaerobic metabolism at moderate swimming speeds. J. Exp. Biol., 97:359-373.

Engäs, A. 1994. The effects oftrawl performance and fish behaviour on the catching efficiency ofdemersal sampling trawls. In: Marine Fish Behaviour in Capture and Abundance Estimation, A. Femö and S. Olsen [eds.]. Oxford: Fishing News Books, p. 45-68.

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Godo,O.R. 1994. Factors affecting reliability ofgroundfish abundance estimates from bottom trawl surveys.ln: Marine Fish Behaviour in Capture and Abundance Estimation, A. Fernö and S. Olsen [eds.]. Oxford: Fishing News Books, p. 166-199. .

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12 .~

) Table 1. A listing ofthe capture dates, locations, and gear types for Alantic cod and American plaice used to support the experiment. Date Species Location of Capture Method

Jun.271996 cod &plaice Grand Bank survey trawl Aug. 14 1996 cod Petty Harbour, Nfld. cod trap Nov.71996 cod Sugarloaf Point, Nfld. jigged Nov. 221996 plaice Grand Bank survey trawl Nov. 231996 plaice Grand Bank survey trawl Nav.251996 plaice Grand Bank survey trawl Dec.71996 plaice Grand Bank survey trawl Feb. 20 1997 cad Narthwest Arm, Trinity Bay, Nfld. jigged Mar. 181997 cod Smith Sound, Trinity Bay, Nfld. long-Iine Mar. 201997 cad Smith Sound, Trinity Bay, Nfld. lang-line Mar.211997 cod Smith Sound, Trinity Bay, Nfld. long-Iine Apr.251997 cod &plaice S1. Pierre Bank survey trawl May 91997 plaice Grand Bank survey trawl Jun. 9 1997 cod Fox Harbour Nfld. cod trao

300or------, 200 100 ~~ '2 20 • :s. 10 ~ j 2 1 :~ .3 • .2 .1 '-- --,.---,.---,.---,.---.....---.....---.,---..... 50 60 70 80 90 1 0 110 140 Swimming Speed (cm/s) Figure 1 Endurance plotted against swirnming speed for all trials ofAtlantic cod, (r =0.674).

13 l 4.0 3.0 • § 2.0 • ~ • • • ::J .".. -. . • • 1.0 •• ••.~ • • • ~ ,,~:.-.. .. • 0.0' . ~ef# •• ... ~ y.. • .•••,. ,....-­•• • ~ '" -1.0 •• • • • • • • CO .~,­ , . . .. • • • •• • ~ -2.0' -.. • -3.0 -4.0 -1.0 0.0 1.0 2.0 3.0 4.0 5~0 6.0 7.0 8.0 10.0 Water Temperature (C) Figure 2 Endurance (with the effect ofswimming speed removed) plotted against water temperature for all trials ofAtlantic eod.

300 200 100 ~ .~~ • ~~ • • -c: 20 • • • :s 10 * ~ • 3 2 I _.Temperature (C) 1 • 6.0 to 9.8 C •••• :~ 4.5 to 5.9 C .3 • .2 --• 1.5 to 2.9 C .1 -• O.Oto 1.4C 50 60 70 80 90 100 110 120 130 140 Swimming Speed (cm/s) Figure 3 Endurance plotted against swimming speed grouped by temperature for all trials ofAtlantic cod.

14 ...

4.0_------. 3.0 • 2.0 • • • • 1.0. 1 ... .:.. : ..: • I· • ••• • •• • J • • •••• • •••• ••• • ~ 0.0. : •• • •• •• 11 •• ••• • • ••• • 1 •••• • • i -1.0 • • tJS :. ••••• ••• ••• • • -2.0. .• . .. .. - -3.0.

-4.0 L----_----.-----..------o.-----_..- ----.I. 1.60 1.li5 1.'0 1.'5 1110 1.135 1.1l0 Ul5

Log10 (Fish Length em) Figure 4 Enduranee (with the effeet ofswimming speed removed) plotted against the logarithm offish length for all trials ofAtlantie eod.

300 200 ...... 100 ...... ~~ .. -c 20 ...... :s 10 ...... ~ ...... 3 ...... J 2 ...... 1 ...... :~ .. .3 .2 .1 15 20 25 30 35 40 45

Fish Length (em) Figure 5 Enduranee plotted against fish length for all trials ofAmeriean plaiee, (r = 0.429).

15

. " 3.0

2.0 ...... ~ ",.. 1.0 ...... •.. .. ~ "... .. I 0.0 .. ~ ~ ...... • ...... ~ .. .. • .. .. ~ .." -1.0 ". .. .. Itl.l ...... ". .. -2.0 ..

-3.0 -1.0 0:0 1:0 2:0 3~0 4~0 5:0 6:0 '1.0 8:0 9:0 1 .0

Water Temperature (C) Figure 6 Endurance (with the effect offish length removed) plotted against water temperature for all trials ofAmerican plaice.

16