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Comparison of fish and shrimp trawls for sampling deep-water estuarine fish in a large coastal river in eastern Australia
Fisheries Research 54 (2002) 409±417
Comparison of ®sh and shrimp trawls for sampling deep-water estuarine ®sh in a large coastal river in eastern Australia R.J. West* Environmental Science, Australian Flora and Fauna Research Centre, University of Wollongong, Wollongong, NSW 2522, Australia Received 6 June 2000; received in revised form 23 October 2000; accepted 7 November 2000
Abstract Shrimp and demersal ®sh trawling gears were compared as methods for sampling the deep-water estuarine ®sh communities in the Clarence River, a large east Australian coastal river. Three sites at three separate locations within the river were sampled quarterly over a 2-year period with both nets. Of the 68 ®sh species captured during the sampling program, 47 were caught with both nets, 10 only in the shrimp trawl and 11 only in the ®sh trawl. Summed over the entire study, more ®sh and slightly more ®sh species were captured with the shrimp trawling gear, but estimates of mean CPUEs (combined over species) and ®sh species diversities were very similar using either net during most seasons and at most sites. However, the nets were highly selective for particular species, with Selenotoca mutifasciata, Mugil cephalus, Liza argentea, Herklotsichthys castlenaui and Acanthopagrus australis caught predominantly with the ®sh trawling gear and Philypnidon grandiceps, Ambassis jacksoniensis, Ambassis marianus and Arius graeffei caught mainly with the shrimp trawling gear. For some species, shrimp trawling gear caught larger numbers of smaller individuals. Although gear type affected species composition, data obtained from either net were useful in discussing the spatial variation in the estuarine ®sh communities. Overall, trawling, particularly with the shrimp trawling nets, offered an excellent monitoring tool for the deep-water estuarine ®shes, although a combination of methods might be required for a comprehensive description of ®sh communities. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Fish trawling; Shrimp trawling; Estuarine ®sh; Australia
1. Introduction Trawling with either commercially available gear (shrimp and ®sh nets) or with purposely built trawls, such as beam trawls, is often the preferred method of sampling ®sh in the deeper waters of estuaries (Blaber et al., 1989; Gray et al., 1990; Hostens and Hamerlynck, 1994; Fraser, 1997; Marshall and Elliott, 1998; Hostens and Mees, 1999; Wakwabi and Mees, 1999; West and Walford, 2000). Consequently, in * Tel.: 61-2-4221-4648; fax: 61-2-4221-4665. E-mail address: [email protected] (R.J. West).
many parts of the world, trawling is regularly used as a monitoring tool to assess the size of estuarine ®sh populations and/or to investigate ®sh community structure (e.g. see Stokesbury et al., 1999). However, in common with many ®shing gears, trawling is a very selective method of catching ®sh. For example, Wantiez (1996) compared the use of shrimp trawls and ®sh trawls in assessing ®sh assemblages in St. Vincent Bay (New Caledonia) and found that the selection of trawling gear in¯uenced both species richness and species composition. He also suggested that while either of the two gear types was useful in investigating the general characteristics of soft-bottom
0165-7836/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 7 8 3 6 ( 0 1 ) 0 0 2 7 7 - 6
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R.J. West / Fisheries Research 54 (2002) 409±417
®sh communities, problems arose when the data were used to describe ®sh assemblages at particular sites or to study the occurrence of individual ®sh species. In the study of demersal ®sh communities in tropical Australia, Wassenberg et al. (1997) compared daytime ®sh trawling to night-time shrimp trawling. They concluded that the different sampling strategies caught a different range of species and a different size range for some species. In a more recent study, Stokesbury et al. (1999) compared the otter trawls used in two large-scale ®sheries monitoring programs carried out by separate agencies in the estuaries of North Carolina (USA). They found differences in the selectivity and ef®ciency of the two types of otter trawls and concluded that the data collected by the agencies were not necessarily comparable and could lead to quite different results. A common conclusion of all these authors is that an understanding of sampling bias is essential when designing any ®sheries research based on trawling methods. The present study arose as part of a ®sheries assessment program carried out between 1988 and 1990 in two large coastal rivers in eastern Australia (see also
West and Gordon, 1994; West and King, 1996; West and Walford, 2000). Selectivity of ®shing gears arose as a major concern of local industry representatives and resulted in the use of two types of trawl nets in the main ®sheries surveys, namely a twin-rigged shrimp (or prawn) trawl net and a large demersal ®sh trawl net. This decision proved costly in terms of the extra effort required in the sampling program, but has provided data that allowed a direct comparison to be made of these two gear types. In this paper, a subset of data corresponding to three regions in the Clarence River is analysed to determine the in¯uence of gear type on the description of ®sh communities in estuarine conditions. 2. Materials and methods Trawl gear trials were carried out in the Clarence River (298250 S 1538220 E, see Fig. 1), which is the largest coastal river in northern New South Wales with an estuarine water area of approximately 90 km2 (West et al., 1985) and a catchment of 22,400 km2
Fig. 1. Map of Clarence River showing the three sites in each of the three regions where shrimp and ®sh trawling gear trials were carried out.
R.J. West / Fisheries Research 54 (2002) 409±417
(Bell and Edwards, 1980). The main channel of the Clarence River branches a number of times along an extensive coastal ¯oodplain and there are two large cut-off bays which form shallow muddy basins. For the purposes of comparing trawling gear, the present analyses concentrates on unpublished data collected for a subset of sites located in three regions of the river; three sites each in Oyster Channel, the Broadwater and South Arm (Fig. 1). The shrimp and ®sh trawling gears were operated from a 9 m long wooden boat (Morning Mist, S. Everson, Clarence River). Shrimp trawling gear consisted of two separate nets, each attached to 3 m booms operated on either side of the trawler. Each shrimp net had two wooden otter boards, a slightly weighted leadline, a 10 m stainless steel headline and a single 180 mm diameter
411
¯oat. Shrimp nets were 40 mm diamond mesh throughout. Diving on the nets indicated that they remained on the bottom and opened to about 1 m height. The demersal ®sh trawling gear (Kent Bollinger, Newcastle, Australia) consisted of a single large net with a 22 m headline, 10 small ¯oats attached to the headline, two wooden otter boards and a weighted leadline. Mesh size of this net varied from 229 mm in the wings, 57±152 mm in the main body and 38 mm in the codend. This net opened to about 3 m height. Two 15 min shots using both gear types were collected every quarter between spring 1988 and winter 1990 at each of the nine sites, resulting in a total of 288 samples. Changing between gear types led to a gap of up to 5 days between comparative samples for the different trawl nets. Numbers and weights of
Table 1 Overall catches for top 25 species taken in the Clarence River using a shrimp trawl net and a ®sh trawl net. Sampling was carried out at three sites at each of the three locations, every quarter for a 2-year period (1988±1990) Species
Shrimp trawl
Fish trawl
A. jacksoniensis H. castlenaui Acanthropagrus australis G. subfasciatus A. marianus A. graeffei M. cephalus Notesthes robusta Potamalosa richmondia Plotosus lineatus Pomatomus saltator L. argentea P. grandiceps Monodactylus argenteus Selenotaca multifasciata Euristhmus lepturus Macquaria novaemaculatus P. fuscus Rhabdosargus sarba Torguigener pleirogramma Aseraggodes macleayanus Argyrosomus hololepidotus Sillago ciliata M. petardi Hypseleotris compressus
40609 1821 5131 7743 11312 3050 34 1308 529 876 806 20 896 287 8 472 262 441 314 292 377 167 79 4 162
1685 19636 14280 5858 1971 633 1943 538 1094 624 500 1018 12 547 699 151 225 36 154 142 3 101 176 236 57
42294 21457 19411 13601 13283 3683 1977 1846 1623 1500 1306 1038 908 834 707 623 487 477 468 434 380 268 255 240 219
33 776
32 537
43 1313
58 77776
57 52856
68 130632
No. of additional species captured No. of additional fish captured Total number of species Total number of fish
Total
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R.J. West / Fisheries Research 54 (2002) 409±417
individual ®sh species, and fork lengths (mm, FL) for selected species, were recorded for each shot. All sampling was carried out in daylight hours within a 2-week period using the same ®shing vessel. Two null hypotheses were tested with analysis of variance (ANOVA): that there were no signi®cant differences in the mean number of ®sh captured between gear types (expressed as catch per unit effort, CPUE) and that there were no signi®cant differences in the mean number of ®sh species captured between gear types. Three factors were used in the ANOVAs: gear type (Shrimp Trawl and Fish Trawl); sampling event (spring, summer, autumn and winter); region (Oyster Channel, Broadwater and South Arm). Signi®cant differences p 0:05 between means were identi®ed using the Student Newman Keuls means comparison test. To investigate the effect of gear type on the collection of ®sh community data, cluster analysis and non-metric multi-dimensional scaling (MDS) were used (Field et al., 1982; Clarke, 1993). Raw data consisted of CPUEs for each species, averaged over the 2-year period and grouped into ``site/
gear type'' combinations 9 sites 2 gear types. A similarity matrix (Bray±Curtis) was formed after root±root transformation of the data. Clusters were produced using a group average, hierarchical sorting strategy and relationships between sites, regions and gear types were examined by dendogram and twodimensional MDS ordination plot. 3. Results 3.1. General Combined over the 2-year period of the study and for both trawling methods, there were 130,632 ®sh and 68 ®sh species captured from the three regions (Table 1). Shrimp trawling caught 77,776 ®sh and 58 ®sh species, including 11 species (made up of 80 individual ®sh) not captured by demersal ®sh trawling. Demersal ®sh trawling caught 52,856 ®sh and 57 species, including 10 species (77 ®sh) not captured by shrimp trawling. Of the 68 species captured overall, 47
Fig. 2. Percentage of total number of each species caught by either shrimp or ®sh trawling, totalled over all sites, regions, seasons and years.
R.J. West / Fisheries Research 54 (2002) 409±417
were captured with both gear types. No species unique to one gear type was captured in high numbers (i.e., >50), however the nets were highly selective for many species (Table 1). This is best illustrated for the selected ®sh species shown in Fig. 2. Selenotoca mutifasciata, Mugil cephalus, Liza argentea, Herklotsichthys castlenaui and Acanthopagrus australis were caught predominantly with the ®sh trawling gear, while Philypnidon grandiceps, Ambassis jacksoniensis, Ambassis marianus and Arius graeffei were caught mainly with the shrimp trawling gear (Fig. 2). The trawling gears appeared to be selective for both ®sh size and depth range of the ®sh species. For many species the shrimp trawling gear, which had the smaller mesh size, caught higher numbers of smaller sized ®sh than the ®sh trawling gear. For example, for the two main economic species captured, A. australis and M. cephalus, there were about three times as many ®sh under 100 mm caught by shrimp
413
trawling than that caught by ®sh trawling (Fig. 3). In addition, the small sized Ambassids (A. jacksoniensis and A. marianus) were also more numerous in the shrimp trawl catches (Fig. 2). Other species appeared to be selected on the basis of their position in the water column. Mid-water and surface ®shes, such as H. castlenaui and S. mutifasciata, were caught mainly by ®sh trawling. Bottom feeders, such as Notesthes robusta and A. graeffei, were captured mainly by shrimp trawling. 3.2. Effect of trawling gear on number of ®sh and ®sh species caught Summed over the entire study, more ®sh and slightly more ®sh species were captured with shrimp trawling gear compared to demersal ®sh trawling gear (Table 1). However, there were only a few signi®cant differences due to gear type found for total ®sh
Fig. 3. Length±frequency plots for: (A) A. australis caught when shrimp trawling; (B) A. australis caught when ®sh trawling; (C) M. cephalus caught when shrimp trawling; (D) M. cephalus caught when ®sh trawling.
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R.J. West / Fisheries Research 54 (2002) 409±417
Fig. 4. Comparison of mean CPUE (S.E.) and mean number of species (S.E.) between shrimp trawling gear (ST) and ®sh trawling gear (FT) for each sampling event and for each region of the Clarence River (see Fig. 1). Signi®cant differences are marked with an asterisk.
abundance (CPUE) and none for ®sh diversity (mean number of species). For CPUE, ANOVA revealed signi®cant interactions between the factors region, season and gear type. Shrimp trawling at South Arm during spring and at Broadwater during winter had signi®cantly higher mean CPUEs than did demersal ®sh trawling at the same sites and in the same season (Fig. 4). The higher CPUEs of the shrimp trawling gear in spring at South Arm sites were due to large catches of P. grandiceps, N. robusta and A. jacksoniensis. Differences in catches between the gear types at Broadwater sites in winter were due to catches of Gerres subfasciatus and the two Ambassid species. All these species tended to be captured in greater numbers with the shrimp trawl compared to the demersal ®sh trawl (Fig. 2). There were fewer differences in mean ®sh diversity (mean number of species) between regions and seasons, and no signi®cant differences due to gear type and no interaction between gear type and other factors (Fig. 4).
3.3. Effect of trawling gear on ®sh community data The effect of trawling gear on the collection of ®sh community data was investigated using hierarchical clustering and MDS analyses of the 18 ``site/gear type'' combinations of data. Three major groupings of these data were formed in the cluster analysis at about 65% similarity (Fig. 5). Oyster Channel sites formed one major cluster, and this cluster included both shrimp trawl and ®sh trawl data. However, the situation was not so clear for the remainder of the samples. These remaining samples formed major clusters based on the type of ®shing gear rather than on the sampling location. Thus, South Arm shrimp trawling data were ®rst associated with Broadwater shrimp trawling data, before separating further on the basis of location (Fig. 5). This demonstrates the effect that sampling bias can have on the interpretation of ®sh community data and reinforces the need to adopt consistent sampling methods when comparing ®sh
R.J. West / Fisheries Research 54 (2002) 409±417
415
Fig. 5. Dendogram showing results of cluster analysis for region/method groupings of ®sh community data. The association matrix was based on the mean catch of each species combined over sampling periods. The data was root±root transformed (see Section 2 for further details).
communities between locations. MDS and ordination provided a more useful interpretation of these data. A two-dimensional plot revealed six major clusters, which were easily distinguished as ``region/gear type'' groupings (Fig. 6). Gear type in¯uenced the position of the clusters in one-dimension (Dimension 2 in Fig. 6), while location within the estuary in¯uenced the position of the clusters in the other (Dimension 1 in Fig. 6). Use of either of the gear types alone would have been suf®cient to separate the three locations (South Arm, Broadwater and Oyster Channel) on the basis of their ®sh community structure (Fig. 6).
Fig. 6. Ordination plot of MDS analyses showing relationship between region/gear type groupings of ®sh community data. The association matrix used was based on the mean catch of each species combined over sampling periods. The data was root±root transformed (see Section 2 for further details).
4. Discussion It is well known that trawling is a very selective method of sampling ®sh communities but there are few alternatives that offer the ease of sampling or the
416
R.J. West / Fisheries Research 54 (2002) 409±417
ef®ciency in terms of catches. As a result of these obvious advantages, trawling is widely adopted for sampling ®sh in the deeper waters of bays and estuaries (Blaber et al., 1989; Gray et al., 1990; Hostens and Hamerlynck, 1994; Fraser, 1997; Wassenberg et al., 1997; Marshall and Elliott, 1998; Hostens and Mees, 1999; Wakwabi and Mees, 1999). However, there is an extensive range of trawling gear used to sample estuarine ®sh communities from small experimental beam trawls (e.g. Marshall and Elliott, 1998) to large commercial ®shing nets (e.g. Fraser, 1997). A number of studies, usually in open ocean waters or large bays, have compared the ®sh catches obtained using different trawling gears (Dealteris et al., 1989; Stender and Barans, 1994; Wantiez, 1996; Wassenberg et al., 1997; Stokesbury et al., 1999). In this study, shrimp trawling and ®sh trawling gears have been directly compared as methods for assessing deep-water estuarine ®sh communities. Both gear types are used commercially in eastern Australian waters, although the use of ®sh trawling gear is banned in estuaries. Surprisingly, the in¯uence of trawling gear on the estimates of overall CPUE was small in this study (Fig. 4) and there was no discernible in¯uence on the estimates of ®sh diversity between locations and seasons (Fig. 4). This was largely because there were only a few ®sh species that were ``unique'' to either one of the gear types and these tended to be species caught in low numbers using either gear type. Forty-seven of the 68 species caught were common to both gear types (Table 1). This also meant that both sets of data (from shrimp trawling or from ®sh trawling) were useful for comparing the structure of ®sh communities at different locations within the river (Fig. 6). However, not unexpectedly, gear type did have a large impact on the catches of individual species (Table 1; Fig. 2) and on the sizes of the ®shes captured (Fig. 3). These effects were most likely the result of gear selectivity caused by using nets of different mesh size, trawling height and herding ability. One of the most useful outcomes of using both gear types in this study was to demonstrate to industry and other community groups that their perceptions about the abundances of particular ®sh species in the estuary was at least partly a result of the selectivity of ®shing methods. This in¯uence is best demonstrated by the two species listed at the top of Table 1. A. jacksoniensis
was by far the most abundant ®sh species captured with shrimp trawling gear, 52% of the total number of ®sh caught, but only 3% of the catch was taken with ®sh trawling gear (Table 1). Likewise, H. castlenaui made up 37% of the number of ®sh captured when ®sh trawling and only 2% of the catch when shrimp trawling (Table 1). Most of the larger economically important ®sh species, such as A. australis, M. cephalus, Liza argenteus, Myxus petardi and Girella tricuspidata, were captured in higher numbers in the ®sh trawl. The exception was the bottom dwelling dusky ¯athead, Platycephalus fuscus, which was caught mainly by shrimp trawling. The results of these gear trials indicate that, used alone, either the shrimp trawl or the ®sh trawl could be expected to give a reasonable description of the deepwater estuarine ®sh communities in these coastal river systems. Both gear types are likely to sample the most common species, including economically important species, and to provide information that allows for spatial comparisons to be made of the ®sh communities at various locations within the rivers. However, the makeup of the catches in terms of overall species composition, number of individual species and size ranges of some species differs between shrimp and ®sh trawl nets. As a result, a detailed description of the estuarine ®sh assemblages would be in¯uenced by the selection of trawl gear. These results are similar to those of Wantiez (1996) who compared the use of ®sh trawling and shrimp trawling in St. Vincent Bay, New Caledonia, and concluded that ``the spatial variations of the community structure were comparable between the two gear types, though species assemblages were not the same''. The view of Wassenberg et al. (1997), who sampled areas in the Great Barrier Reef (Australia), that ``neither gear type can be used on its own for an adequate description of the ®sh community'' might also be supported for the estuarine ®sh communities sampled in eastern Australia. Another factor that needs to be considered in selecting ®shing gear is the physical size of the trawling gear as this places restrictions on the sampling program. Even though the coastal river sampled in this study was large, there were many sites where ®sh trawling gear could not be used, either because the river channels were too narrow or the water too shallow. In fact, demersal ®sh trawling even in these large estuaries was cumbersome and sometimes dangerous
R.J. West / Fisheries Research 54 (2002) 409±417
due to the small size of the trawler. Considering the small amount of additional information obtained and the dif®culty in operation, the usefulness of commercially available ®sh trawling gear for routine monitoring of estuaries is limited. Overall, the choice of ®shing gear used to sample ®sh in an estuary is dependent on the objectives of the study and the size of the waterbody. For example, in biodiversity studies, obviously a combination of a large number of ®shing methods are required for a complete inventory of estuarine ®sh species. Likewise, where a particular species is to be targeted, selectivity of ®shing gear needs to be carefully considered. However, the results of this study suggest that trawling, particularly with shrimp trawling nets, offers a good monitoring tool for deep-water estuarine ®shes and should provide a useful measure of relative abundance for the majority of the ®sh species present. Acknowledgements Field work was completed while the author was in NSW Fisheries, Australia. The assistance of Philip Gibbs, Trudy Walford, Tracey McVea, Glen Cuthbert, Keith Chilcott, Matt Broadhurst and Steven Everson was greatly appreciated. References Bell, F.C., Edwards, A.R., 1980. An environmental inventory of estuaries and coastal lagoons in New South Wales. Total Environment Centre, Sydney, 187 pp. Blaber, S.J.M., Brewer, D.T., Salini, J.P., 1989. Species composition and biomasses of ®shes in different habitats of a northern Australian estuary: their occurrence in the adjoining sea and estuarine dependence. Estuar. Coast. Shelf Sci. 29, 509±531. Clarke, K.R., 1993. Non-parametric multivariate analyses of changes in community structure. Aust. J. Ecol. 18, 117±143. Dealteris, J.T., Recksiek, C.W., Fahfouhi, A., Liuxiong, X., 1989. Comparison of the performance of two bottom-sampling trawls. Trans. Am. Fish. Soc. 118, 119±130.
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Field, J.G., Clarke, K.R., Warwick, R.M., 1982. A practical strategy for analysing mutispecies distribution patterns. Mar. Ecol. Prog. Ser. 8, 37±52. Fraser, T.H., 1997. Abundance, seasonality, community indices, trends and relationships with physicochemical factors of trawled ®sh in upper Charlotte Harbor. Florida Bull. Mar. Sci. 60, 739±763. Gray, C.A., McDonnall, V.C., Reid, D.D., 1990. By-catch from prawn trawling in the Hawkesbury River, New South Wales: species composition, distribution and abundance. Aust. J. Mar. Fresh. Res. 41, 13±26. Hostens, K., Hamerlynck, O., 1994. The mobile epifauna of the soft bottoms in the subtidal Oosterschelde Estuary Ð structure, function and impact of the storm-surge barrier. Hydrobiologia 283, 479±496. Hostens, K., Mees, J., 1999. The mysid-feeding guild of demersal ®shes in the brackish zone of the Westerschelde Estuary. J. Fish Biol. 55, 704±719. Marshall, S., Elliott, M., 1998. Environmental in¯uences on the ®sh assemblage of the Humber Estuary, UK. Estuar. Coast. Shelf Sci. 46, 175±184. Stender, B.W., Barans, C.A., 1994. Comparison of the catch from tongue and two-seam shrimp nets off South Carolina. N. Am. J. Fish. Manage. 14, 178±195. Stokesbury, K.D.E., Bichy, J.B., Ross, S.W., 1999. Selectivity and ef®ciency of two otter trawls used to assess estuarine ®sh and macroinvertebrate populations in North Carolina. Estuaries 22, 882±888. Wakwabi, E.O., Mees, J., 1999. The epibenthos of the backwaters of a tropical mangrove creek (Tudor creek, Mombasa, Kenya). Neth. J. Zool. 49, 189±206. Wantiez, L., 1996. Comparison of ®sh assemblages sampled by a shrimp trawl and a ®sh trawl, in St. Vincent Bay, New Caledonia. J. Mar. Biol. Assoc. UK 76, 759±775. Wassenberg, T.J., Blaber, S.J.M., Burridge, C.Y., Brewer, D.T., Salini, J.P., Gribble, N., 1997. The effectiveness of ®sh and shrimp trawls for sampling ®sh communities in tropical Australia. Fish. Res. 30, 241±251. West, R.J., Gordon, G.N.G., 1994. Commercial and recreational harvest of ®sh from two Australian coastal rivers. Aust. J. Mar. Fresh. Res. 45, 1259±1279. West, R.J., King, R.J., 1996. Marine, brackish, and freshwater ®sh communities in the vegetated and bare shallows of an Australian coastal river. Estuaries 19, 31±41. West, R.J., Walford, T.R., 2000. Estuarine ®shes in two large eastern Australian coastal rivers Ð does prawn trawling in¯uence ®sh community structure? Fish. Manage. Ecol. 7, 523±536. West, R.J., Thorogood, C., Walford, T., Williams, R.J., 1985. An estuarine inventory for New South Wales, Australia. New South Wales Department of Agriculture and Fisheries, Sydney, 140 pp.
Comparison of ®sh and shrimp trawls for sampling deep-water estuarine ®sh in a large coastal river in eastern Australia R.J. West* Environmental Science, Australian Flora and Fauna Research Centre, University of Wollongong, Wollongong, NSW 2522, Australia Received 6 June 2000; received in revised form 23 October 2000; accepted 7 November 2000
Abstract Shrimp and demersal ®sh trawling gears were compared as methods for sampling the deep-water estuarine ®sh communities in the Clarence River, a large east Australian coastal river. Three sites at three separate locations within the river were sampled quarterly over a 2-year period with both nets. Of the 68 ®sh species captured during the sampling program, 47 were caught with both nets, 10 only in the shrimp trawl and 11 only in the ®sh trawl. Summed over the entire study, more ®sh and slightly more ®sh species were captured with the shrimp trawling gear, but estimates of mean CPUEs (combined over species) and ®sh species diversities were very similar using either net during most seasons and at most sites. However, the nets were highly selective for particular species, with Selenotoca mutifasciata, Mugil cephalus, Liza argentea, Herklotsichthys castlenaui and Acanthopagrus australis caught predominantly with the ®sh trawling gear and Philypnidon grandiceps, Ambassis jacksoniensis, Ambassis marianus and Arius graeffei caught mainly with the shrimp trawling gear. For some species, shrimp trawling gear caught larger numbers of smaller individuals. Although gear type affected species composition, data obtained from either net were useful in discussing the spatial variation in the estuarine ®sh communities. Overall, trawling, particularly with the shrimp trawling nets, offered an excellent monitoring tool for the deep-water estuarine ®shes, although a combination of methods might be required for a comprehensive description of ®sh communities. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Fish trawling; Shrimp trawling; Estuarine ®sh; Australia
1. Introduction Trawling with either commercially available gear (shrimp and ®sh nets) or with purposely built trawls, such as beam trawls, is often the preferred method of sampling ®sh in the deeper waters of estuaries (Blaber et al., 1989; Gray et al., 1990; Hostens and Hamerlynck, 1994; Fraser, 1997; Marshall and Elliott, 1998; Hostens and Mees, 1999; Wakwabi and Mees, 1999; West and Walford, 2000). Consequently, in * Tel.: 61-2-4221-4648; fax: 61-2-4221-4665. E-mail address: [email protected] (R.J. West).
many parts of the world, trawling is regularly used as a monitoring tool to assess the size of estuarine ®sh populations and/or to investigate ®sh community structure (e.g. see Stokesbury et al., 1999). However, in common with many ®shing gears, trawling is a very selective method of catching ®sh. For example, Wantiez (1996) compared the use of shrimp trawls and ®sh trawls in assessing ®sh assemblages in St. Vincent Bay (New Caledonia) and found that the selection of trawling gear in¯uenced both species richness and species composition. He also suggested that while either of the two gear types was useful in investigating the general characteristics of soft-bottom
0165-7836/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 7 8 3 6 ( 0 1 ) 0 0 2 7 7 - 6
410
R.J. West / Fisheries Research 54 (2002) 409±417
®sh communities, problems arose when the data were used to describe ®sh assemblages at particular sites or to study the occurrence of individual ®sh species. In the study of demersal ®sh communities in tropical Australia, Wassenberg et al. (1997) compared daytime ®sh trawling to night-time shrimp trawling. They concluded that the different sampling strategies caught a different range of species and a different size range for some species. In a more recent study, Stokesbury et al. (1999) compared the otter trawls used in two large-scale ®sheries monitoring programs carried out by separate agencies in the estuaries of North Carolina (USA). They found differences in the selectivity and ef®ciency of the two types of otter trawls and concluded that the data collected by the agencies were not necessarily comparable and could lead to quite different results. A common conclusion of all these authors is that an understanding of sampling bias is essential when designing any ®sheries research based on trawling methods. The present study arose as part of a ®sheries assessment program carried out between 1988 and 1990 in two large coastal rivers in eastern Australia (see also
West and Gordon, 1994; West and King, 1996; West and Walford, 2000). Selectivity of ®shing gears arose as a major concern of local industry representatives and resulted in the use of two types of trawl nets in the main ®sheries surveys, namely a twin-rigged shrimp (or prawn) trawl net and a large demersal ®sh trawl net. This decision proved costly in terms of the extra effort required in the sampling program, but has provided data that allowed a direct comparison to be made of these two gear types. In this paper, a subset of data corresponding to three regions in the Clarence River is analysed to determine the in¯uence of gear type on the description of ®sh communities in estuarine conditions. 2. Materials and methods Trawl gear trials were carried out in the Clarence River (298250 S 1538220 E, see Fig. 1), which is the largest coastal river in northern New South Wales with an estuarine water area of approximately 90 km2 (West et al., 1985) and a catchment of 22,400 km2
Fig. 1. Map of Clarence River showing the three sites in each of the three regions where shrimp and ®sh trawling gear trials were carried out.
R.J. West / Fisheries Research 54 (2002) 409±417
(Bell and Edwards, 1980). The main channel of the Clarence River branches a number of times along an extensive coastal ¯oodplain and there are two large cut-off bays which form shallow muddy basins. For the purposes of comparing trawling gear, the present analyses concentrates on unpublished data collected for a subset of sites located in three regions of the river; three sites each in Oyster Channel, the Broadwater and South Arm (Fig. 1). The shrimp and ®sh trawling gears were operated from a 9 m long wooden boat (Morning Mist, S. Everson, Clarence River). Shrimp trawling gear consisted of two separate nets, each attached to 3 m booms operated on either side of the trawler. Each shrimp net had two wooden otter boards, a slightly weighted leadline, a 10 m stainless steel headline and a single 180 mm diameter
411
¯oat. Shrimp nets were 40 mm diamond mesh throughout. Diving on the nets indicated that they remained on the bottom and opened to about 1 m height. The demersal ®sh trawling gear (Kent Bollinger, Newcastle, Australia) consisted of a single large net with a 22 m headline, 10 small ¯oats attached to the headline, two wooden otter boards and a weighted leadline. Mesh size of this net varied from 229 mm in the wings, 57±152 mm in the main body and 38 mm in the codend. This net opened to about 3 m height. Two 15 min shots using both gear types were collected every quarter between spring 1988 and winter 1990 at each of the nine sites, resulting in a total of 288 samples. Changing between gear types led to a gap of up to 5 days between comparative samples for the different trawl nets. Numbers and weights of
Table 1 Overall catches for top 25 species taken in the Clarence River using a shrimp trawl net and a ®sh trawl net. Sampling was carried out at three sites at each of the three locations, every quarter for a 2-year period (1988±1990) Species
Shrimp trawl
Fish trawl
A. jacksoniensis H. castlenaui Acanthropagrus australis G. subfasciatus A. marianus A. graeffei M. cephalus Notesthes robusta Potamalosa richmondia Plotosus lineatus Pomatomus saltator L. argentea P. grandiceps Monodactylus argenteus Selenotaca multifasciata Euristhmus lepturus Macquaria novaemaculatus P. fuscus Rhabdosargus sarba Torguigener pleirogramma Aseraggodes macleayanus Argyrosomus hololepidotus Sillago ciliata M. petardi Hypseleotris compressus
40609 1821 5131 7743 11312 3050 34 1308 529 876 806 20 896 287 8 472 262 441 314 292 377 167 79 4 162
1685 19636 14280 5858 1971 633 1943 538 1094 624 500 1018 12 547 699 151 225 36 154 142 3 101 176 236 57
42294 21457 19411 13601 13283 3683 1977 1846 1623 1500 1306 1038 908 834 707 623 487 477 468 434 380 268 255 240 219
33 776
32 537
43 1313
58 77776
57 52856
68 130632
No. of additional species captured No. of additional fish captured Total number of species Total number of fish
Total
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individual ®sh species, and fork lengths (mm, FL) for selected species, were recorded for each shot. All sampling was carried out in daylight hours within a 2-week period using the same ®shing vessel. Two null hypotheses were tested with analysis of variance (ANOVA): that there were no signi®cant differences in the mean number of ®sh captured between gear types (expressed as catch per unit effort, CPUE) and that there were no signi®cant differences in the mean number of ®sh species captured between gear types. Three factors were used in the ANOVAs: gear type (Shrimp Trawl and Fish Trawl); sampling event (spring, summer, autumn and winter); region (Oyster Channel, Broadwater and South Arm). Signi®cant differences p 0:05 between means were identi®ed using the Student Newman Keuls means comparison test. To investigate the effect of gear type on the collection of ®sh community data, cluster analysis and non-metric multi-dimensional scaling (MDS) were used (Field et al., 1982; Clarke, 1993). Raw data consisted of CPUEs for each species, averaged over the 2-year period and grouped into ``site/
gear type'' combinations 9 sites 2 gear types. A similarity matrix (Bray±Curtis) was formed after root±root transformation of the data. Clusters were produced using a group average, hierarchical sorting strategy and relationships between sites, regions and gear types were examined by dendogram and twodimensional MDS ordination plot. 3. Results 3.1. General Combined over the 2-year period of the study and for both trawling methods, there were 130,632 ®sh and 68 ®sh species captured from the three regions (Table 1). Shrimp trawling caught 77,776 ®sh and 58 ®sh species, including 11 species (made up of 80 individual ®sh) not captured by demersal ®sh trawling. Demersal ®sh trawling caught 52,856 ®sh and 57 species, including 10 species (77 ®sh) not captured by shrimp trawling. Of the 68 species captured overall, 47
Fig. 2. Percentage of total number of each species caught by either shrimp or ®sh trawling, totalled over all sites, regions, seasons and years.
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were captured with both gear types. No species unique to one gear type was captured in high numbers (i.e., >50), however the nets were highly selective for many species (Table 1). This is best illustrated for the selected ®sh species shown in Fig. 2. Selenotoca mutifasciata, Mugil cephalus, Liza argentea, Herklotsichthys castlenaui and Acanthopagrus australis were caught predominantly with the ®sh trawling gear, while Philypnidon grandiceps, Ambassis jacksoniensis, Ambassis marianus and Arius graeffei were caught mainly with the shrimp trawling gear (Fig. 2). The trawling gears appeared to be selective for both ®sh size and depth range of the ®sh species. For many species the shrimp trawling gear, which had the smaller mesh size, caught higher numbers of smaller sized ®sh than the ®sh trawling gear. For example, for the two main economic species captured, A. australis and M. cephalus, there were about three times as many ®sh under 100 mm caught by shrimp
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trawling than that caught by ®sh trawling (Fig. 3). In addition, the small sized Ambassids (A. jacksoniensis and A. marianus) were also more numerous in the shrimp trawl catches (Fig. 2). Other species appeared to be selected on the basis of their position in the water column. Mid-water and surface ®shes, such as H. castlenaui and S. mutifasciata, were caught mainly by ®sh trawling. Bottom feeders, such as Notesthes robusta and A. graeffei, were captured mainly by shrimp trawling. 3.2. Effect of trawling gear on number of ®sh and ®sh species caught Summed over the entire study, more ®sh and slightly more ®sh species were captured with shrimp trawling gear compared to demersal ®sh trawling gear (Table 1). However, there were only a few signi®cant differences due to gear type found for total ®sh
Fig. 3. Length±frequency plots for: (A) A. australis caught when shrimp trawling; (B) A. australis caught when ®sh trawling; (C) M. cephalus caught when shrimp trawling; (D) M. cephalus caught when ®sh trawling.
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Fig. 4. Comparison of mean CPUE (S.E.) and mean number of species (S.E.) between shrimp trawling gear (ST) and ®sh trawling gear (FT) for each sampling event and for each region of the Clarence River (see Fig. 1). Signi®cant differences are marked with an asterisk.
abundance (CPUE) and none for ®sh diversity (mean number of species). For CPUE, ANOVA revealed signi®cant interactions between the factors region, season and gear type. Shrimp trawling at South Arm during spring and at Broadwater during winter had signi®cantly higher mean CPUEs than did demersal ®sh trawling at the same sites and in the same season (Fig. 4). The higher CPUEs of the shrimp trawling gear in spring at South Arm sites were due to large catches of P. grandiceps, N. robusta and A. jacksoniensis. Differences in catches between the gear types at Broadwater sites in winter were due to catches of Gerres subfasciatus and the two Ambassid species. All these species tended to be captured in greater numbers with the shrimp trawl compared to the demersal ®sh trawl (Fig. 2). There were fewer differences in mean ®sh diversity (mean number of species) between regions and seasons, and no signi®cant differences due to gear type and no interaction between gear type and other factors (Fig. 4).
3.3. Effect of trawling gear on ®sh community data The effect of trawling gear on the collection of ®sh community data was investigated using hierarchical clustering and MDS analyses of the 18 ``site/gear type'' combinations of data. Three major groupings of these data were formed in the cluster analysis at about 65% similarity (Fig. 5). Oyster Channel sites formed one major cluster, and this cluster included both shrimp trawl and ®sh trawl data. However, the situation was not so clear for the remainder of the samples. These remaining samples formed major clusters based on the type of ®shing gear rather than on the sampling location. Thus, South Arm shrimp trawling data were ®rst associated with Broadwater shrimp trawling data, before separating further on the basis of location (Fig. 5). This demonstrates the effect that sampling bias can have on the interpretation of ®sh community data and reinforces the need to adopt consistent sampling methods when comparing ®sh
R.J. West / Fisheries Research 54 (2002) 409±417
415
Fig. 5. Dendogram showing results of cluster analysis for region/method groupings of ®sh community data. The association matrix was based on the mean catch of each species combined over sampling periods. The data was root±root transformed (see Section 2 for further details).
communities between locations. MDS and ordination provided a more useful interpretation of these data. A two-dimensional plot revealed six major clusters, which were easily distinguished as ``region/gear type'' groupings (Fig. 6). Gear type in¯uenced the position of the clusters in one-dimension (Dimension 2 in Fig. 6), while location within the estuary in¯uenced the position of the clusters in the other (Dimension 1 in Fig. 6). Use of either of the gear types alone would have been suf®cient to separate the three locations (South Arm, Broadwater and Oyster Channel) on the basis of their ®sh community structure (Fig. 6).
Fig. 6. Ordination plot of MDS analyses showing relationship between region/gear type groupings of ®sh community data. The association matrix used was based on the mean catch of each species combined over sampling periods. The data was root±root transformed (see Section 2 for further details).
4. Discussion It is well known that trawling is a very selective method of sampling ®sh communities but there are few alternatives that offer the ease of sampling or the
416
R.J. West / Fisheries Research 54 (2002) 409±417
ef®ciency in terms of catches. As a result of these obvious advantages, trawling is widely adopted for sampling ®sh in the deeper waters of bays and estuaries (Blaber et al., 1989; Gray et al., 1990; Hostens and Hamerlynck, 1994; Fraser, 1997; Wassenberg et al., 1997; Marshall and Elliott, 1998; Hostens and Mees, 1999; Wakwabi and Mees, 1999). However, there is an extensive range of trawling gear used to sample estuarine ®sh communities from small experimental beam trawls (e.g. Marshall and Elliott, 1998) to large commercial ®shing nets (e.g. Fraser, 1997). A number of studies, usually in open ocean waters or large bays, have compared the ®sh catches obtained using different trawling gears (Dealteris et al., 1989; Stender and Barans, 1994; Wantiez, 1996; Wassenberg et al., 1997; Stokesbury et al., 1999). In this study, shrimp trawling and ®sh trawling gears have been directly compared as methods for assessing deep-water estuarine ®sh communities. Both gear types are used commercially in eastern Australian waters, although the use of ®sh trawling gear is banned in estuaries. Surprisingly, the in¯uence of trawling gear on the estimates of overall CPUE was small in this study (Fig. 4) and there was no discernible in¯uence on the estimates of ®sh diversity between locations and seasons (Fig. 4). This was largely because there were only a few ®sh species that were ``unique'' to either one of the gear types and these tended to be species caught in low numbers using either gear type. Forty-seven of the 68 species caught were common to both gear types (Table 1). This also meant that both sets of data (from shrimp trawling or from ®sh trawling) were useful for comparing the structure of ®sh communities at different locations within the river (Fig. 6). However, not unexpectedly, gear type did have a large impact on the catches of individual species (Table 1; Fig. 2) and on the sizes of the ®shes captured (Fig. 3). These effects were most likely the result of gear selectivity caused by using nets of different mesh size, trawling height and herding ability. One of the most useful outcomes of using both gear types in this study was to demonstrate to industry and other community groups that their perceptions about the abundances of particular ®sh species in the estuary was at least partly a result of the selectivity of ®shing methods. This in¯uence is best demonstrated by the two species listed at the top of Table 1. A. jacksoniensis
was by far the most abundant ®sh species captured with shrimp trawling gear, 52% of the total number of ®sh caught, but only 3% of the catch was taken with ®sh trawling gear (Table 1). Likewise, H. castlenaui made up 37% of the number of ®sh captured when ®sh trawling and only 2% of the catch when shrimp trawling (Table 1). Most of the larger economically important ®sh species, such as A. australis, M. cephalus, Liza argenteus, Myxus petardi and Girella tricuspidata, were captured in higher numbers in the ®sh trawl. The exception was the bottom dwelling dusky ¯athead, Platycephalus fuscus, which was caught mainly by shrimp trawling. The results of these gear trials indicate that, used alone, either the shrimp trawl or the ®sh trawl could be expected to give a reasonable description of the deepwater estuarine ®sh communities in these coastal river systems. Both gear types are likely to sample the most common species, including economically important species, and to provide information that allows for spatial comparisons to be made of the ®sh communities at various locations within the rivers. However, the makeup of the catches in terms of overall species composition, number of individual species and size ranges of some species differs between shrimp and ®sh trawl nets. As a result, a detailed description of the estuarine ®sh assemblages would be in¯uenced by the selection of trawl gear. These results are similar to those of Wantiez (1996) who compared the use of ®sh trawling and shrimp trawling in St. Vincent Bay, New Caledonia, and concluded that ``the spatial variations of the community structure were comparable between the two gear types, though species assemblages were not the same''. The view of Wassenberg et al. (1997), who sampled areas in the Great Barrier Reef (Australia), that ``neither gear type can be used on its own for an adequate description of the ®sh community'' might also be supported for the estuarine ®sh communities sampled in eastern Australia. Another factor that needs to be considered in selecting ®shing gear is the physical size of the trawling gear as this places restrictions on the sampling program. Even though the coastal river sampled in this study was large, there were many sites where ®sh trawling gear could not be used, either because the river channels were too narrow or the water too shallow. In fact, demersal ®sh trawling even in these large estuaries was cumbersome and sometimes dangerous
R.J. West / Fisheries Research 54 (2002) 409±417
due to the small size of the trawler. Considering the small amount of additional information obtained and the dif®culty in operation, the usefulness of commercially available ®sh trawling gear for routine monitoring of estuaries is limited. Overall, the choice of ®shing gear used to sample ®sh in an estuary is dependent on the objectives of the study and the size of the waterbody. For example, in biodiversity studies, obviously a combination of a large number of ®shing methods are required for a complete inventory of estuarine ®sh species. Likewise, where a particular species is to be targeted, selectivity of ®shing gear needs to be carefully considered. However, the results of this study suggest that trawling, particularly with shrimp trawling nets, offers a good monitoring tool for deep-water estuarine ®shes and should provide a useful measure of relative abundance for the majority of the ®sh species present. Acknowledgements Field work was completed while the author was in NSW Fisheries, Australia. The assistance of Philip Gibbs, Trudy Walford, Tracey McVea, Glen Cuthbert, Keith Chilcott, Matt Broadhurst and Steven Everson was greatly appreciated. References Bell, F.C., Edwards, A.R., 1980. An environmental inventory of estuaries and coastal lagoons in New South Wales. Total Environment Centre, Sydney, 187 pp. Blaber, S.J.M., Brewer, D.T., Salini, J.P., 1989. Species composition and biomasses of ®shes in different habitats of a northern Australian estuary: their occurrence in the adjoining sea and estuarine dependence. Estuar. Coast. Shelf Sci. 29, 509±531. Clarke, K.R., 1993. Non-parametric multivariate analyses of changes in community structure. Aust. J. Ecol. 18, 117±143. Dealteris, J.T., Recksiek, C.W., Fahfouhi, A., Liuxiong, X., 1989. Comparison of the performance of two bottom-sampling trawls. Trans. Am. Fish. Soc. 118, 119±130.
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Field, J.G., Clarke, K.R., Warwick, R.M., 1982. A practical strategy for analysing mutispecies distribution patterns. Mar. Ecol. Prog. Ser. 8, 37±52. Fraser, T.H., 1997. Abundance, seasonality, community indices, trends and relationships with physicochemical factors of trawled ®sh in upper Charlotte Harbor. Florida Bull. Mar. Sci. 60, 739±763. Gray, C.A., McDonnall, V.C., Reid, D.D., 1990. By-catch from prawn trawling in the Hawkesbury River, New South Wales: species composition, distribution and abundance. Aust. J. Mar. Fresh. Res. 41, 13±26. Hostens, K., Hamerlynck, O., 1994. The mobile epifauna of the soft bottoms in the subtidal Oosterschelde Estuary Ð structure, function and impact of the storm-surge barrier. Hydrobiologia 283, 479±496. Hostens, K., Mees, J., 1999. The mysid-feeding guild of demersal ®shes in the brackish zone of the Westerschelde Estuary. J. Fish Biol. 55, 704±719. Marshall, S., Elliott, M., 1998. Environmental in¯uences on the ®sh assemblage of the Humber Estuary, UK. Estuar. Coast. Shelf Sci. 46, 175±184. Stender, B.W., Barans, C.A., 1994. Comparison of the catch from tongue and two-seam shrimp nets off South Carolina. N. Am. J. Fish. Manage. 14, 178±195. Stokesbury, K.D.E., Bichy, J.B., Ross, S.W., 1999. Selectivity and ef®ciency of two otter trawls used to assess estuarine ®sh and macroinvertebrate populations in North Carolina. Estuaries 22, 882±888. Wakwabi, E.O., Mees, J., 1999. The epibenthos of the backwaters of a tropical mangrove creek (Tudor creek, Mombasa, Kenya). Neth. J. Zool. 49, 189±206. Wantiez, L., 1996. Comparison of ®sh assemblages sampled by a shrimp trawl and a ®sh trawl, in St. Vincent Bay, New Caledonia. J. Mar. Biol. Assoc. UK 76, 759±775. Wassenberg, T.J., Blaber, S.J.M., Burridge, C.Y., Brewer, D.T., Salini, J.P., Gribble, N., 1997. The effectiveness of ®sh and shrimp trawls for sampling ®sh communities in tropical Australia. Fish. Res. 30, 241±251. West, R.J., Gordon, G.N.G., 1994. Commercial and recreational harvest of ®sh from two Australian coastal rivers. Aust. J. Mar. Fresh. Res. 45, 1259±1279. West, R.J., King, R.J., 1996. Marine, brackish, and freshwater ®sh communities in the vegetated and bare shallows of an Australian coastal river. Estuaries 19, 31±41. West, R.J., Walford, T.R., 2000. Estuarine ®shes in two large eastern Australian coastal rivers Ð does prawn trawling in¯uence ®sh community structure? Fish. Manage. Ecol. 7, 523±536. West, R.J., Thorogood, C., Walford, T., Williams, R.J., 1985. An estuarine inventory for New South Wales, Australia. New South Wales Department of Agriculture and Fisheries, Sydney, 140 pp.