Density effects on juvenile shortfinned eel (Anguilla australis) cover preferences in replicate channels

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1 New Glova Density Zealand Journal effects of on Marine juvenile and eel Freshwater cover preferences Research, 2002, Vol. 36: /02/ $7.00 The Royal Society of New Zealand Density effects on juvenile shortfinned eel (Anguilla australis) cover preferences in replicate channels G. J. GLOVA National Institute of Water and Atmospheric Research Limited P.O. Box 8602 Christchurch, New Zealand Abstract The effects of density on day-time cover preferences of juvenile ( mm total length) shortfinned eels (Anguilla australis Richardson) were tested in replicate channels provided with natural (cobbles, macrophytes, woody debris) and artificial (shade, plastic pipes) cover during summer. The densities used were 10, 20, 30, 40, 50, 60, 80, 100, and 200 eels channel 1 (each channel = 2.4 m 2 ), with each density tested in triplicate, or better. Eels used cobbles and macrophytes almost exclusively, with cobbles being the most preferred and their use increased with increasing density; however, at 200 eels channel 1, escape behaviour was evident during the night. In tests at 100 eels channel 1, with only woody debris, plastic pipes, and shade sections available, the eels occupied woody debris, pipes, and shade in decreasing order. In further tests at 100 eels channel 1, with cobbles, macrophytes and woody debris included, but with plastic pipes partially buried in the substrate and shade sections lowered to within 20 mm of the bottom, still only cobbles and macrophytes were used. Finally, in tests of 10, 50, and 100 eels channel 1, with cover in each channel consisting of cobbles, macrophytes, woody debris, fallen sod (clumps of natural stream bank with riparian grasses), and modified shade (i.e., lowered to within 20 mm of the bottom with macrophyte rootlets attached to underside), eels preferred cobbles, shade, and fallen sod in decreasing order at all three densities. Overall, the findings of this study do not lend strong support to the hypothesis that juvenile eels use a greater variety of cover with increasing population density. M01049; published 17 September 2002 Received 22 June 2001; accepted 20 December 2001 Keywords juvenile shortfinned eels; Anguilla australis; cover preferences; population density; replicate channels INTRODUCTION The distribution and habitat associations of wild populations of the two main species of freshwater eels in New Zealand (Anguilla australis Richardson and A. dieffenbachii Gray) have received considerable attention in recent years (Chisnall 1996; Glova et al. 1998; Jellyman & Chisnall 1999). Subsequently, these works were extended to laboratory studies in replicate channels in an effort to determine the effects of eel size and species interactions, and presence of large longfins, on juvenile eel cover preferences (Glova 1999, 2001). The emphasis in these studies was on juvenile eels as it was surmised that their populations are largely limited by the availability of suitable day-time cover. A major gap remaining in our understanding of juvenile eel ecology is the effect of population density on cover preferences, particularly as eel densities can vary significantly with recruitment (Vollestad & Jonsson 1988) and available habitat (Glova et al. 1998). Presumably, in response to increased population pressure, eels use suboptimal cover or habitat in addition to those most preferred. Although there are no data for Anguilla species for this hypothesis, data do exist for other fish species. As examples, Greenberg (1994) found that juvenile brown trout (Salmo trutta) use of pools and marginal areas of runs (considered to be suboptimal) in artificial streams increased with population density. Bult et al. (1999) noted that a greater proportion of Atlantic salmon (Salmo salar) parr occupied pools than runs at higher population densities in stream enclosures. Rangeley & Kramer (1998) observed that juvenile pollock (Pollachius virens) made significant habitat shifts with changes in population density in intertidal enclosures. The objective in the present study was to determine the effects of density on juvenile shortfinned

2 484 New Zealand Journal of Marine and Freshwater Research, 2002, Vol. 36 eel cover preferences in replicate channels in which eel sizes and numbers could be controlled. It was hypothesised that eels would use a greater variety of cover types with increasing population density. The study consists of two parts. The first, is a systematic approach to determine the effects of increasing eel density on the basic cover types used by Glova (1999, 2001) in previous experiments in replicate channels; the second involves some manipulation of cover to determine eel use of alternative cover types (Glova et al. 1998) at different densities. It was intended that the results of the latter would have wider management implications for eel populations in streams. Because of the large numbers of eels required in these tests and current lack of juvenile longfinned eels generally in New Zealand streams (Jellyman et al. 2000), the present study was limited to shortfinned eels. METHODS Replicate channels The channels, site, and water supply of the present study were the same as those described by Glova (1999), so only a brief account of the facility is given here. Basically, the setup consisted of nine replicate wooden channels (each 4.7 m long 0.5 m wide 0.3 m deep), screened at both ends and sealed on the bottom with a sheet of perforated black plastic. The channels were placed in raceways with a flow of c. 15 litres s 1, which provided an average water depth and velocity in the channels of 0.15 m and 0.03 m s 1, respectively. Overlaying the black plastic on the bottom was a layer of sand and fine gravel, over which the different cover types were placed. The experiments were run between 1 February and 19 March 1999, during which the water temperature ranged from 15.8 to 18.3 C. Collecting and holding of eels Eels used in the experiments were of the medium size range ( mm total length, TL) used by Glova (1999), and were collected from the lower reach of the nearby Cust River at the beginning of the study with a pulsed DC backpack electroshocker. Additional collections from the same river were made later in the study because of the large number (maximum 600) of eels required per week as densities were increased. As a result, a proportion of the eels was reused in later experiments as it was not possible to collect such large numbers of fresh eels from the same population. The eels were held at moderate densities in live-boxes placed in raceways and fed weekly an abundance of freshly caught benthic invertebrates (mostly larval mayflies, caddisflies, and dipterans) from the Cust River. On completion of the study the eels were returned to their site of capture. Experimental procedure The eel densities and cover types used in experiments on juvenile shortfinned eel cover preferences are given in Fig. 1. A random block design was used in locating the cover types in the channels, although complete randomisation was not possible because of the various logistical problems discussed by Glova (1999). In the first part of the study (cover unmodified), the main cover types were macrophytes, woody debris, cobbles, shade, and plastic pipes (Glova 1999) and will only be briefly described here. The macrophyte used was exclusively watercress (Rorippa nasturtiumaquaticum), taken fresh from the Cust River for each test (average wet weight channel 1 = 3.5 kg). The woody debris was composed of a mixture of small tree branches and bark with adequate periphytic growth (average wet weight channel 1 = 4.2 kg). The cobbles formed a layer of stones (median size 78 mm) c. 150 mm deep, laid on top of the sand and gravel bed. The shade sections were made of finely perforated black plastic underlay (c. 90% light reduction), fastened to wooden frames that floated on the water surface. The pipes consisted of seven PVC sections c. 300 mm long (diam. range mm), arranged in a staggered pattern on the bed. These five cover types located in different segments in the channels (Fig. 1, Experiments 1 5) were tested at 10, 20, 30, 40, 50, 60, 80, 100, and 200 eels channel 1, with six replicates being done for each of the first three densities and three replicates for all others. Initially, it was planned to limit the tests to a maximum of 30 eels channel 1, but as the tests progressed it was realised that much higher densities were required to attempt to alter eel cover preferences. In the second part of the study, tests were carried out in triplicate at 10, 50, and 100 eels channel 1 (Fig. 1, Experiments 6 8) with some manipulation/ modification of cover. First, watercress and cobbles (the preferred cover by juvenile eels) were removed from the suite of five main cover types and only woody debris, shade, and plastic pipes were available. Second, watercress and cobbles were reintroduced, and, based on the assumption that body contact is important in eel use of cover (Glova 1999),

3 Glova Density effects on juvenile eel cover preferences 485 Fig. 1 Diagram showing arrangement of cover types and eel densities in experiments conducted in the replicate channels (1 = upstream end). Cover abbreviations: CB = cobbles; WC = watercress; SH = shade; WD = woody debris; PP = plastic pipes; PPB = plastic pipes buried; SHL = shade lowered; FS = fallen sod; SHLM = shade lowered with macrophytes under. In Experiment No. 6, watercress and cobbles were excluded and the arrangement of the three remaining cover types (i.e., PP, SH, and WD) was interchanged between replicates so as to test each of them in the three segments shown. the shade sections were lowered to within 40 mm of the bottom and the plastic pipes were buried with only the downstream ends protruding from the bed. Third, the plastic pipes were replaced with fallen sod (clumps of stream bank with riparian grasses, size c m 2 ), and macrophyte rootlets were stapled to the underside of the lowered shade sections to simulate the texture of natural undercut banks. The test procedure was the same as that described by Glova (1999), with eels released into the channels on a Monday and the test terminated on Friday. The cover arrangement for each test was set up on the previous Friday, allowing the weekend for conditions in the channels to stabilise. For further procedural details, refer to Glova (1999). Data analysis Eel counts, tabulated by experiment, replicate number, distance (segment) in the channels, and cover type were set up on a spreadsheet. The counts were converted to percentages (to standardise the results between channels) by summing the counts of eels recovered at the end of a test within each channel and dividing the counts in each cover type by the sum of eels in the channel. The proportional data were not transformed, but rather the means and variances were plotted to examine the effects of density on juvenile shortfinned eel cover preferences. A generalised linear model using Poisson loglikelihood functions (McCullagh & Nelder 1989) was fitted to the eel counts using log-transformed generalised linear directives (Genstat 5 Committee 1993). The analysis was run separately on the data series for the unmodified and modified cover experiments. Possible upstream/downstream bias in the distribution of juvenile shortfinned eels in the channels was assumed to be similar to that reported by Glova (1999) and therefore was not specifically tested in the present study. Rather, the data of the unmodified cover experiments were pooled and analysed for possible upstream/downstream bias and the findings were used to adjust for bias in the distribution of eels in the channels. In the analysis, the cover preference results were adjusted for distance effects and conversely the distances favoured by eels were adjusted for cover effects. The cover and distance effects were jointly estimated, with the analysis separating out the two effects on the results obtained. Basically, for cover types that occurred more often at the distances favoured by eels, the results were adjusted downward, whereas for those occurring less often at favoured distances, the results were adjusted upward. In the analysis of the results of the modified cover experiments, the data could not be adjusted for bias because each of the cover types was not tested in all channel segments. For a more detailed overall account of the application of the model, refer to Glova (1999). RESULTS Bias in longitudinal distribution of eels The effect of longitudinal distance on the distribution of eels in the channels was significant (P < 0.001; Table 1). Plotting the average percentage of eels by channel segment for these experiments (Fig. 2) shows there was a bias in eel distribution, with the

4 486 New Zealand Journal of Marine and Freshwater Research, 2002, Vol. 36 upstream end preferred over the downstream end. In particular, there was a pronounced preference for the second segment from the upstream end, whereas the downstream end of the channels was least preferred. Density effects on eel cover preferences unmodified cover series At densities ranging between 10 and 200 eels channel 1, juvenile shortfinned eels virtually exclusively occupied cobbles and watercress in the unmodified cover test series (Fig. 3). The exception was a single eel present in woody debris at a density of 100 eels channel 1 ; plastic pipes and shade were completely unoccupied over this range of densities. For the densities tested overall, mean values of 48.8 ± 3.1% (SE) and 51.1 ± 3.1% of the eels occupied watercress and cobbles, respectively. However, the relative use of watercress and cobbles changed significantly (P < 0.001) with eel density (Fig. 3). At between 10 and 30 eels channel 1, both covers were preferred almost equally, whereas at eels channel 1, eels preferred watercress (65 88%) over cobbles. In contrast, at eels channel 1, cobbles were preferred (54 85%) over watercress. In one of the channels at the maximum density (i.e., 200 eels channel 1 ), 20% of the eels escaped by climbing up the walls and getting through the netting over the top of the channel. Cover was the most significant factor in the distribution of eels in these experiments (as indicated by the high deviance ratio, 471.4), and interacted significantly with density (Table 1). Since cobbles and watercress were the only covers used by eels in these experiments, the results are summarised by plotting the percentage of eels in cobbles at the different densities (Fig. 4). With the exception of the relatively low percentages of eels in cobbles at densities of eels channel 1, there is a trend of Fig. 2 Percentages (mean ± SE) of juvenile shortfinned eels (Anguilla australis) per channel section (1 = upstream end) based on the combined data set for Experiments 1 5 inclusive in the replicate channels. Table 1 Generalised linear model analysis based on counts of juvenile shortfinned eel (Anguilla australis) in relation to cover type in the unmodified and modified cover series in the replicate channels. Refer to Fig. 1 for reference to experiment numbers. Mean Deviance Chi-square Variable d.f. Deviance deviance ratio probability Unmodified cover series (Expt no. 1 5) Channel <0.001 Cover <0.001 Cover density <0.001 Distance <0.001 Residual Modified cover series (Expt no. 7) Channel Cover <0.001 Residual Modified cover series (Expt no. 8) Channel <0.001 Cover <0.001 Cover density Residual

5 Glova Density effects on juvenile eel cover preferences 487 Fig. 3 Distribution of juvenile shortfinned eels (Anguilla australis) in relation to cover (unmodified) at densities between 10 and 200 eels channel 1 in the replicate channels. Data plotted are means (horizontal line within boxes), one standard errors (boxes), and minima and maxima (vertical lines) of the proportion of eels present in each cover (symbols are the same as in Fig. 1) at the end of the test period. the remainder were in shade (Fig. 5, left), but only in the channel in which shade was in the most upstream segment. A large proportion (65%) of the eels escaped from one of the three channels, in the manner described earlier. Fig. 4 Percentages (mean ± SE) of juvenile shortfinned eels (Anguilla australis) occupying cobbles at densities between 10 and 200 eels channel 1 in the replicate channels. increasing use of cobbles with increasing density of eels. Of significance is that the variance around the means decreases with increasing density, which is indicative of greater consistency in eel cover preference as densities increase. Density effects on eel cover preferences modified cover series Cobbles and watercress excluded In experiments at 100 eels channel 1 without cobbles and watercress, on average c. 75% of the eels occupied woody debris, 23% chose plastic pipes, and Pipes partially buried, shade lowered In further experiments at 100 eels channel 1, but with the plastic pipes partially buried and shade sections lowered to near the bottom (Fig. 5, right), the eels still preferred cobbles (77.5 ± 3.1%) and watercress (21.1 ± 3.0%), although the difference between them was highly significant (Table 1). Only two eels occupied pipes and single eels were present in both the woody debris and shade. In one of the channels, 10% of the eels escaped. Fallen sod, shade lowered with macrophytes beneath In tests at densities of 10, 50, and 100 eels channel 1, with the plastic pipes replaced with fallen sod and macrophyte rootlets stapled to the underside of the lowered shade sections, the cover types preferred at all three densities in decreasing order were cobbles (64.8 ± 4.4%), shade (23.3 ± 3.9%), and fallen sod (8.9 ± 2.6%) (Fig. 6). In contrast, the percentages of eels in woody debris and watercress were 0.9 ± 0.9% and 2.2 ± 1.3%, respectively. There was no significant interaction between cover and density in these tests (Table 1).

6 488 New Zealand Journal of Marine and Freshwater Research, 2002, Vol. 36 Fig. 5 Distribution of juvenile shortfinned eels (Anguilla australis) in relation to cover (modified) tested at a density of 100 eels channel 1 in the replicate channels. For symbols of cover types and data plots see Fig. 1 and 3, respectively. Fig. 6 Distribution of juvenile shortfinned eels (Anguilla australis) in relation to cover (modified) at densities of 10, 50, and 100 eels channel 1 in the replicate channels. For symbols of cover types and data plots see Fig. 1 and 3, respectively. DISCUSSION Density effects on eel cover preferences The results of the present study do not lend strong support to the hypothesis that juvenile eels use a greater variety of cover with increasing population density. At densities ranging from 10 to 200 eels channel 1, juvenile shortfinned eels continued to occupy cobbles and macrophytes almost exclusively. Juvenile eels have been found to congregate at very high densities in preferred cover in the wild (Jellyman & Glova 1998). The relatively high proportion of escapees recorded from some of the channels at densities of 100 and 200 eels channel 1 in the present study, suggests that the eels strongly rejected the other cover types available to them (i.e., woody debris, shade, and plastic pipes). Escape behaviour was not recorded in cover preference tests of juvenile shortfinned and longfinned eels at densities of eels channel 1 (Glova 1999), nor when large predatory longfinned eels were included in the tests (Glova 2001). The occurrence of such behaviour in the present study was probably a response to overcrowding in preferred cover (cobbles and macrophytes). On few occasions at night, small eels (<120 mm long) were seen crawling in the doublelayered lids of the channels (chicken wire mesh with windbreak cloth over), attempting to escape. It appears that juvenile shortfinned eels tolerate greater crowding in cobbles than macrophytes. At low to moderate densities (10 30 eels channel 1 ), cobbles and macrophytes were preferred about equally, as was found by Glova (1999), but at exceedingly high densities (say >80 eels channel 1 ), cobbles were preferred over macrophytes. A possible explanation for this is that the interstices of cobbles can accommodate higher eel densities than can the more open structure of macrophytes. Although aggression has been found to be low in juvenile eels (Knights 1987; Glova & Jellyman 2000), some physical separation between individuals may be

7 Glova Density effects on juvenile eel cover preferences 489 essential and is more likely to occur in cobbles than macrophytes at high densities. The distribution of juvenile shortfinned eels in streams is not strongly associated with in-stream debris (Glova et al. 1998), most probably because it is favoured by large longfinned eels (Taylor 1988) whose presence may have an avoidance effect on smaller eels (Chisnall & Hicks 1993; Glova 2001). In the wild, juvenile eels tend to favour the interstices of cobbles in riffles and runs (Glova et al. 1998; Sagar & Glova 1998). Since the use of woody debris did not increase with eel density in the present study, it appears that juvenile shortfinned eels have a low preference for woody debris. Only when woody debris was the only natural cover available, was it occupied to a significant extent by eels. Preference for natural versus artificial cover Artificial cover (shade, plastic pipes) was virtually unused by juvenile shortfinned eels, irrespective of population density. Although plastic pipes were largely rejected, in aquaria with no other cover available, juvenile eels do use them (Graynoth 1998). The removal of preferred natural cover (cobbles, macrophytes) had limited effect on increasing the use of artificial cover. Only when artificial cover was modified to include natural elements (macrophyte rootlets attached to the underside of shade sections), was it accepted to a moderate degree. A possible explanation is that the modified shade sections resembled the character of undercut banks, which are preferred sites of eels (Taylor 1988; Glova et al. 1998). Body contact with the cover occupied appears to be of importance to eels and other fish. As examples, juvenile eels have been reported to occupy small brush piles wrapped in wire mesh placed on barren, sandy areas of shallow lakes (Jellyman & Chisnall 1999). In the laboratory, juvenile eels have been noted to be intensely packed into a single pipe, while others were unoccupied (Tesch 1977). Juvenile brown trout (S. trutta) in experimental channels have been reported (DeVore & White 1978) to prefer overhead cover near the bottom with plastic ribbons attached beneath, as opposed to overhead cover higher in the water column with no ribbons. CONCLUDING REMARKS From the findings of this study it is concluded that juvenile shortfinned eels have relatively narrow cover preferences that are not greatly altered by population density. Their frequent use of cobbles in the replicate channels is consistent with their distribution in lowland streams (Glova et al. 1998; Sagar & Glova 1998). Macrophytes, although frequently used by small eels in experimental conditions (Glova 1999, present study), are not occupied to a great extent in the wild (Glova et al. 1998), probably because of the presence of large eels (Glova 2001). Enhancement of cover in streams with natural (e.g., macrophytes) and semi-natural (e.g., undercut banks with tactual elements) components is more likely to be used by eels than if only artificial cover is provided (e.g., clay pipes). ACKNOWLEDGMENTS Of NIWA, Christchurch, I am grateful to T. Gough and D. H. Lucas for technical assistance in all phases of the experimental work; D. Maindonald for reliable and methodical maintenance of the channels throughout the experimental period; M. L. Bonnett for data collation and computer graphic presentations; and D. Jellyman and B. McDowall for comments on a draft version of the manuscript. Also, I sincerely thank D. Baird of AgResearch, Lincoln, for conducting the log-linear statistical analysis on the data set using the Genstat 5 programme. This study was funded by the Foundation for Research, Science and Technology (New Zealand) Contract C REFERENCES Bult, T.; Riley, S. C.; Haedrich, R. L.; Gibson, R. J.; Heggenes, J. 1999: Density-dependent habitat selection by juvenile Atlantic salmon (Salmo salar) in experimental riverine habitats. Canadian Journal of Fisheries and Aquatic Sciences 56: Chisnall, B. L. 1996: Habitat associations of juvenile shortfinned eels (Anguilla australis) in shallow Lake Waahi, New Zealand. New Zealand Journal of Marine and Freshwater Research 30: Chisnall, B. L.; Hicks, B. J. 1993: Age and growth of longfinned eels (Anguilla dieffenbachii) in pastoral and forested streams in the Waikato River basin, and in two hydroelectric lakes in the North Island, New Zealand. New Zealand Journal of Marine and Freshwater Research 27: DeVore, P. W.; White, R. J. 1978: Daytime responses of brown trout (Salmo trutta) to cover stimuli in stream channels. Transactions of the American Fisheries Society 107: Genstat 5 Committee 1993: Genstat 5 Release 3 reference manual. Oxford, Clarendon Press.

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