Comparison between a fine mesh trap net and five ...

New Zealand Journal of Marine and Freshwater Research, 1989, Vol. 23: 321-324 0028-8330/2303-0321$2.50/0 © Crown copyright 1989

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Comparison between a fine mesh trap net and five other fishing gears for sampling shallow-lake fish communities in New Zealand (Note) JOHN W. HAYES Fisheries Research Centre Ministry? of Agriculture and Fisheries, P.O. Box 6016, Rotorua, New Zealand Present address: Freshwater Fisheries Centre, Ministry of Agriculture and Fisheries, P.O. Box 8324, Riccarton, Christchurch, New Zealand Abstracit This paper describes the first tests of a fine mesh trap net suitable for sampling shallowlake fish communities in New Zealand. The catch from 2 shallow lower Waikato lakes of this and 5 other gear types frequently used in freshwater fisheries research in New Zealand are compared. The trap net caught the widest range of fish species and sizes, and was the least selective for species composi tion and relative abundance of any individual gear type. A combination of trap net and gill net caught the widest range of species and sizes for individual or pairwise combinations of gear types, and is recommended as being the best combination for sampling shallow-lake fish communities in New Zealand. Keywords trap nets; selectivity; shallow water; fish; community composition; environmental impact assessment; species inventory INTRODUCTION Community composition and structure are the biological indicators most often used in aquatic environmental impact assessment (Hellawell 1986). However, sampling of fish communities can be complicated by the wide variation in size and mobility and behaviour of fishes. Sampling efficiency favours

Received 8 December 1988; accepted 31 March 1989

a combination involving the fewest possible gear types covering the widest range of species and sizes whilst also minimising sampling effort. This paper describes a fine mesh trap net (modified from Beamish 1973) which, of 6 gear types tested, came closest to meeting the above requirements for environmental impact assessment and inventory surveys of shallow-lake fish communities in New Zealand. Small trap nets are commonly used in North America for fish inventories and monitoring, and their construction, performance, and advantages over other gear types have been documented by several authors (e.g.,Crowe 1950; Patriarche 1968; Beamish 1972,1973; Yeh 1977; Laarman & Ryckman 1982; Hamley & Howley 1985). The present paper describes the first test of such gear in New Zealand. METHODS AND MATERIALS Trap net The trap net was similar to that described by Beamish (1973), but was modified to improve its durability and ease of setting and recovery. Modifications are described below and the reader is referred to Beamish (1972,1973) for details of design and construction. The wings and pot sections of the modified trap net were constructed from 1.0 mm stretched mesh, opaque polyester netting, and the leader was made from 2.0 mm stretched mesh, grey fibreglass netting. The polyester netting is very durable but expensive (c. NZ$25 per m at the time of writing). As considerable strain is exerted on the wings and pot sections during setting, and especially during recovery, a strong durable material is necessary to avoid mesh distortion and tearing. Fibreglass netting was used on the remainder of the net for economy (only c. NZ$4 per m at the time of writing) but this material perishes quickly, and easily tears after stressing. In more recent sampling, polyester netting has proven to be a better leader material than fibrglass netting. A double pot design was used to separate small fish from large piscivorous eels. Each pot was 1 m3

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and fish entrances into each pot were 10 cm2. The fish entrance of the second pot was covered with 1 cm mesh to exclude large eels. The lid of each pot was fastened on 3 sides with "Velcro" and domes, allowing the entire lid to be peeled back for easy removal of the catch. I modified the 15 X 1 m leader by making it detachable from the heart of the trap with the use of "Velcro" and dome fastenings, and with clips and loops attached to the float and lead lines. The float and lead lines were strung inside the 50 mm vinyl hem of the netting. A detachable cover and 1 m tapered extension leader converted the net for fishing water deeper than 1 m (Beamish 1973). The trap net was set from the bow of a 4.9 m aluminium boat, "pot end first"; this usually took c. 10 min. The wings and leader were folded in a concertina fashion to allow orderly entry into the water as the boat was reversed. The pot and leader ends of the net were secured with Danforth anchors. The wings were initially set with concrete setting anchors which were later replaced with Danforth anchors when the net was tightened during final positioning. Setting was best carried out parallel with the wind, and was difficult to achieve in strong winds ( > c. 25 knots). The net was recovered "leader end first" after the anchors were removed. The leader (and cover if present) was detached before the wings and heart of the trap were brought to the side of the boat Fish were either removed with dip nets while the pots rested in the water, or they were poured into a bucket.

Other gear types Five gear types were used for comparison with the trap net. These were: Gee minnow traps (6 mm2 mesh), fyke nets (12 mm stretched mesh), gill nets, a 10 m fine mesh (4 mm stretched mesh) beach seine, and a 20 X 3 m fine mesh (2 mm stretched mesh) purse seine. The fyke nets were single-throated with an entry hoop diameter of 600 mm, a throat diameter of 100 mm, and were fished with a 2.6 m single leader. The gill nets were composed of three 10 X 2 m panels of 55, 80, and 100 mm stretched mesh monofilament and one 5 X 2 m panel of 30 mm stretched mesh. Apart from the purse seine, all of the above gear types are frequently used in freshwater fisheries research in New Zealand. Sampling Sampling was carried out in Lakes Waahi and Whangape (37°34'S, 175°08'E and 37°28'S, 175°04'E, respectively) which are shallow lakes (mean depths 2.0 and 1.5 m, respectively) in the lower Waikato River catchment, North Island. One site on each lake was sampled on 22 January 1986 and again on 20 February 1987. At each site 1 trap net, 3 Gee minnow traps, 1 fyke net (omitted once at 1 site), and 1 or 2 gill nets were set overnight for 18 h, and up to 3 hauls each were made with the beach and purse seines. The catch for each gear type was sorted by species, counted, and the species fork length ranges

Table 1 Number of each species said their length ranges (mm) caught by 6 gear types in Lakes Whangape and Waahi in January 1986 and February 1987; number of samples (net nights or hauls) in parentheses. Gee minnow (Afcl2) No. New Zealand natives Shoitfinned eel. Anguilla australis 52 Common bully, Gobiomorpkus colidianus 2 Common smelt. Relropinna retropinna Iiianga, Galaxias maculatus Grey mullet, Mugil cephalus Exotics Mosquito fish, Gambusia qffinis Goldfish, Carassius auratus Catfish, Ictalwus nebulosus Rudd, Scardinius erythrophthalmus

Length

Fyke net (Af=3) No.

Length

Trap net (N=4) No. Length

Gill net (N=6) No.

Length

Beach seine (N=8)

Purse seine

No.

Length

No. Length

100

120-450

139

>120

1378

>70

47—49

240

30-66

3688

15-80

351

15-57

20-110 45-135

82 128

44-75 55-65

10 21-81 1 70-95

378 24 43

16-45 23-95 30-50

10 4

105 162

2 9-63

184 220-425

3

848 15 13

15-55 10-190 65-280

16 140-275 40 160-500

5 134-200

7-44 12-45

Hayes—Fishing gears for shallow-lake fish communities were recorded. Catches for each gear type were then pooled. RESULTS AND DISCUSSION Nine species of fish were caught with the 6 gear types (Table 1). The trap net and beach seine each caught the greatest number of species and the former was the hast selective for species composition and relative abundance. The beach seine, being a small active fishing gear, could be expected to sample small littoral fish (e.g., common bullies, common smelt, manga, and mosquito fish) more representatively than most passive gear types (Nielsen & Johnson 1983). The trap net catch most closely approximated the relative proportions of these fish caught by the beach seine. However, the trap net had advantages over the beach seine in that it could be fished continuously, thereby damping diurnal variation in fish abundance; and it successfully sampled eels which easily evaded capture by beach seine. The high selectivity of fyke nets and particularly of Gee minnow traps was no doubt related to their small size but also to their relatively large mesh size (see below). Gill nets selected for large midwaterswimming fish, particularly grey mullet. They were the only gear that caught grey mullet which appear to be relatively abundant in the study lakes. These fish are frequently seen, often in large shoals, and they support a commercial fishery. However, being timid and relatively strong swimmers, they avoided the small seines used in the study. Their timidity and possibly reluctance to follow a leader (pers. obs.) would al so explain why they are rarely captured by trap or fyke net. Gear types were also selective for fish size, with selectivity mainly related to mesh size but also, as discussed above, to fish behaviour. The trap net and purse seine caught the smallest individuals of most species, and the trap net caught the widest range of fish sizes (Table 1). Apart from eels, the largest fish were caught by gill nets. Small fish such as common smelt and inanga can be caught in small mesh, fine monofilament gill nets but as these nets are easily torn they are impracticable for repeated sampling. The pair-wise combination of gear types which caughttliegreatestnumberof species and individuals with the widest range of sizes was that of trap nets with gill nets. These 2 gear types present the best combination for sampling shallow-lake fish communities.

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It would be unwise to pool the catch from trap nets and gill nets to describe community composition and structure because their unit efforts are not comparable. For environmental impact assessment the catches from these 2 gear types could be used to compare between areas and times within gear type. Before this could be done a decision must be made on which catch should be used for those species caught by both gear types. Generally, trap nets consistently caught the smaller and gill nets the larger individuals of these species. For example, in this study catfish and goldfish were caught by both trap nets and gill nets (Table 1) but each species can be separated by size into the respective gear types (Table 1). For catfish > 160 mm and goldfish > 140 mm the gill net catch could be used, with the trap net catch being used for smaller fish. Since this study, I have successfully used a combination of trap nets and gill nets for comparing the fish communities of Lakes Waahi and Whangape in an environmental impact study (Hayes unpubl. data). Furthermore, this additional sampling effort, and that from other lakes in North Island, has substantiated the suitability of trap nets for capturing a wide range of species for inventory surveys. To date I have caught 12 species with trap nets. In addition to those species listed for trap nets in Table 1 these are: longfinned eel (Anguilla dieffenbachii), grey mullet, giant kokopu (Galaxias argenteus), rudd and juvenile rainbow trout (Oncorhynchus mykiss, formerly Salmo gairdneri). I have occasionally caught larger catfish, goldfish, and grey mullet with trap nets than those recorded for this gear type in the present study. Also I have used trap nets to qualitatively sample the mysid shrimp Tenagomysis c fultoni in Lakes Whangape and Waahi allowing comparisons to be made on relative abundance between lakes (Chapman et al. 1988; Hayes unpubl. data). The trap nets also appeared to sample fish successfully in submerged macrophyte beds. Although they are more difficult to set in such conditions they caught the same species and similar numbers of fish as those caught in nearby open water (Hayes unpubl. data). When set, the trap nets flatten submerged macrophytes, but in very dense stands the wings and leader may need to be positioned by a diver to ensure the lead lines are on the bottom. The effectiveness of trap nets in capturing a wide range of species and sizes with comparatively low selectivity outweighs their greater expense and handling effort over other gear types available in New Zealand. They are less limited by habitat than

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some of the active gear types (e.g., beach seines), and they obtain larger samples than the smaller passive gear types (e.g., Gee minnow traps and some fyke nets), making them better suited to sampling large water bodies. GDI net catches complement those of trap nets. Jointly these 2 gear types capture the widest range of species and sizes and they present the best combination for sampling shallowlake fish communities.

ACKNOWLEDGMENTS I thank B. L. Chisnall, T. Johns, and M. J. Rutledge for assisting with gear construction and operation, and D. J. Jellyman and R. M. McDowall for constructive criticism of the manuscript.

REFERENCES Beamish, R. J. 1972: Design of a trap net for sampling shallow-water habitaXs.FisheriesResearchBoard of Canada technical report 305.14 p. 1973: Design of a trap net with interchangeable parts for the capture of large and small fishes from varying depths. Journal of the Fisheries Research Board of Canada 30: 587-590.

Chapman, M. A.; Lasenby, D. L.; Hayes, J. W. 1988: Tenagomysis chiltoni and fish in the Waikato Region in New Zealand: some consequences of environmental disturbance. Paper presented to a Mysid-fisheries symposium, American Fisheries Society 118th Annual Meeting, 12-15 September 1988, Toronto, Ontario. Crowe, W. R. 1950: Construction and use of small trap nets. The progressive fish-culturist 12: 185-192. Hamley, J. M.; Howley, T. P. 1985: Factors affecting variability of trap net catches. Canadian journal of fisheries and aquatic sciences 42:1079-1087. Hellawell, J. M. 1986: Biological indicators of freshwater pollution and environmental management. (Pollution monitoring Series.) London, Elsevier Applied Science Publishers Ltd, 546 p. Laarman, P. W.; Ryckman, J. R. 1982: Relative size selectivity of trap nets for eight species of fish. North Americanjournal of fisheries management 2: 33-37. Nielsen, L. A.; Johnson, D. L. 1983: Fisheries techniques. American Fisheries Society, Bethesda, Maryland, USA, 488 p. Patriarche, M. H. 1968: Rate of escape of fish from trap nets. Transactions of the American Fisheries Society 97: 59-61. Yeh, C. F. 1977: Relative selectivity of fishing gear used in a large reservoir in Texas. Transactions of the American Fisheries Society 106: 309-313.