effect of design modifications and soak time variations ...

BULLETIN OF MARINE SCIENCE. 56(2): 475-489,

1995

EFFECT OF DESIGN MODIFICATIONS AND SOAK TIME VARIATIONS ON ANTILLEAN-Z FISH TRAP PERFORMANCE IN A TROPICAL ESTUARY Marcus John Sheaves ABSTRACT Design and operational aspects of Antillean-Z fish traps were investigated under tropical estuarine conditions. Catch rates were compared between different tidal states, mesh sizes, entrance funnel designs, soak times, trap volumes, bait container designs and parts of one lunar cycle. The size structures of major fish species trapped were compared between small and large mesh sizes. Catches were found to be reduced at high tide. Traps with the small (12.5 mm) square mesh produced higher catch rates than those with large (42 mm) hexagonal mesh. The smaller mesh traps retained smaller fishes and captured fewer large sparids (two species) than did the larger mesh traps. Higher catch rates occurred in traps with straight, conical entrances during short (2-h) trap soaks than during long (Ph-d) soaks, while trap catches with horse-head entrance funnels did not differ significantly between the two soak times. This probably reflects greater ease of trap escapement from straight funnels once the initial high attractiveness of trap bait has declined. For most species, more fishes were captured during short (2-h) than long (Ph-d) soaks, indicating that short soaks may be advantageous in many situations. Larger trap sizes (volume approx. 0.92 m3) produced higher catches than smaller trap sizes (volume approx. 0.40 m3) for one species only, Lutjanus russel/i, suggesting that for many species the logistic advantages of smaller traps may make them a useful sampling option, Of two bait container designs tested one with more numerous but smaller holes was found to be advantageous. With this design there was always bait rcmaining in thc container when the trap was censused, ensuring that all traps were baited throughout the soak. The different responses of particular taxa to various trap design modifications and operation investigated in this study suggests that all aspects of trap design and operation need to be considered when designing particular trap sampling programs.

Traps have been increasingly used as reef fish sampling tools in many areas of the world during the last 20 years (e.g., Munro, 1974; Stevenson and StuartSharkey, 1980; Kulbicki and Mou-Tham, 1987; Recksiek et aI., 1991), and more recently within Australia (Whitelaw et a\., 1991). In reef studies, fish traps sample a wide range of species in habitats inaccessible by other techniques due to depth (e.g., visual census), or high levels of structural complexity (e.g., various netting techniques) (Prabhu, 1954). Recently fish traps have been used to sample fishes in a northern Australian estuary (Sheaves, 1992). Here fish traps were used to sample the structurally complex habitats provided by areas of submerged timber under conditions of high turbidity that prohibited the use of visual census. This study also demonstrated the ability of fish traps to sample a number of different habitat/location combinations simultaneously, Studies in reef environments around the world have found that various modifications in fish trap design and mode of operation produce a wide range of effects on trap catches. The influence of mesh sizes on fish trap catches has been investigated in different parts of the world (Wolf and Chis lett, 1974; Stevenson and Stuart-Sharkey, 1980; Luckhurst and Ward, 1987; Ward, 1988; Bohnsack et aI., 1990). The mesh size producing the highest overall catch has generally been different for each particular study, Furthermore, these studies have found that the optimum mesh size for capture of particular species differs, This suggests that the "best" mesh size needs to be determined for particular target taxa in specific 475

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BULLETIN OF MARINE SCIENCE. VOL. 56, NO.2, 1995

locations. Other s.tudies have investigated the effect of varying soak time (the length of time over which the traps are fished) on trap catches (Munro et aI., 1971; Munro, 1974; Wolf and Chislett, 1974; Powles and Barans, 1980; Stevenson and Stuart-Sharkey, 1980; Dalzell and Aini, 1987; Whitelaw et aI., 1991). These studies reported a range of effects including changes in the species composition and catch rates with increasing soak time. As fish trapping is an underutilized sampling technique in estuarine studies it is important to obtain detailed information about the performance of traps in this environment and to develop traps suited to the particular requirements of sampling under estuarine conditions. For example, to sample shallow areas of estuaries efficiently it is necessary to use small craft of shallow draft. This clearly necessitates the use of traps small enough and of suitable shape to be carried in and operated from a small boat. It is necessary also to determine which combinations of trap designs and sampling protocols are best suited to sampling the particular species present in estuaries. In this study, a number of design and operational aspects of Antillean-Z fish traps were investigated under tropical estuarine conditions. The specific aims were to compare catch rates between a) tidal states (high, mid, low), b) two mesh sizes (12.5 and 42 mm), c) two entrance funnel designs (straight and horse-head), d) short (2-h) and long (Ph-d) trap soaks, e) Z-traps of two sizes (0.40 and 0.92 m3), f) two bait container designs and g) different parts of one lunar cycle. Additional aims were to compare size structures of major fish species caught in small and large mesh sizes, and to compare bait retention in two bait container designs. MATERIALS

AND METHODS

The study was conducted at Alligator Creek, a mangrove-lined estuary on the eastern coast of tropical Australia (approx, 19°20'S, 146°55'E), between 20 April and 5 August 1991. A detailed description of Alligator Creek can be found in Sheaves (1992). Trapping was confined to areas of submerged trees and other structural heterogeneity which have been shown to produce high trap catch rates (Sheaves, 1992), The navigable part of the creek was divided into 300 m sections, For each soak, four traps (1 of each combination of mesh size and funnel design) were allocated to a different randomly selected 300 m section, with two sets of four traps being used on each day, Within each section traps were placed in a different, randomly selected order. Traps were oriented in such a way thaI one of the entrance funnels faced down current and one up current. Both small and large Z-traps were used, The small Z-traps were 1,125 mm long, 780 mm wide and 600 mm high, with a surface area of approx. 0.67 m2 and a volume of approx, 0.40 m3, Design details of these traps can be found in Sheaves (1992). The large Z-traps were similar but slightly more than twice the size (I,800 mm long, ],100 mm wide and 600 mm high, plan area of approx. 1.53 m2, volume of approx. 0.92 m3), Before the commenc~:ment of each trap soak, each trap was baited with approximately 500 g of crushed, Western Australian Blue Pilchards (Sardinops neopilchardus). The pilchards were placed in a bait container suspended in the centre of the trap. The bait containers were constructed from 80 mm diam PVC pipe closed off with end caps, Bait pots were of two designs. One was 32 em long with 14, 38 mm diam holes ,~venly spaced around its sides (design A) and the other 245 mm long with 40, 25 mm holes around the sides (design B). Phase I.-Phase 1 of the study was carried out between 20 April and 10 May 1991 using small Z-traps and design A bait pots. Two trap mesh sizes (12.5 mm square and 42 mm hexagonal), two trap entrance funnel designs (horse-head and straight funnels) and two soak times (short-2 hand long-Ph days) were compared. The horse-head entrance funnels were similar in design to those described by Munro (1986) while the straight funnels were simple tapering, laterally compressed, conical tubes of mesh. Each trap had two funnels of the same design, one set into the concave angle of the "Z" on each side. Both funnel designs were placed in such a way that the outer openings were flush with the vertical sides of the traps. The inner openings were towards the mid-lines of the traps, in the case of the straight funnels with the inner entrance vertical and perpendicular to the trap floor, and in the case of the horse-head funnels with the inner aperture horizontal and parallel to the trap floor. Both funnel types had the following approximate dimensions: 400 X 180 mm outer opening, 260 X 150 mm inner op'~ning and 280 mm long.

SHEAVES:FISHTRAPPERFORMANCE

477

TwO-HOUR SOAKS. Three soaks of 2 h duration were made during daylight hours every other day between 20 April and 10 May 1991. The timing of these soaks was arranged to correspond approximately to high, mid and low tides. The factors of "day," "tidal state," "mesh size" and "funnel dcsign" were compared using factorial analysis of variance, with the variates being the total numbers of fish and the numbers of each of the three most commonly trapped species of fish, Acanthopagrus australis, Acanthopagrus berda, and Lutjanus russelli. COMPARISONOF SHORT (2-H) and Long (Ph-d) Trap Soaks. Eight Ph-d soaks were made between 20 April and 10 May. The duration of each soak was from evening of one day to morning 2 days later. For each of the long (¥>-d) soaks, 2-h daytime trapping preceded the beginning of the long soak. For these days the means for the three, 2-h daytime trap soaks were calculated, for each trap. The means for the short soaks were compared to the I ¥>-d trap soak data using factorial analyses of variance. The factors analysed were "length of soak," "day," "mesh size" and "funnel design." The variates compared were total numbers of fish and the numbers of A. australis. A. herda, Epinephelus spp., and L. russelli. For each of the four taxa trapped most commonly, A. australis. A. berda, L. russelli and Epinephelus spp. (the result of pooling the data for E. coioides and E. malabaricus), the data from the two Z-trap funnel designs were pooled for each of the two mesh sizes (12.5 mm and 42 mm) and the size structures compared between the two mesh sizes. The data from both the short and long soaks were included. Where appropriate the two sample Kolmogorov-Smirnov (K-S) test was used to test the significance of the comparisons. Phase 2.-Phase 2 of the study was conducted over one lunar cycle, with two 3-h trap soaks made on each of 2 consecutive days after new moon (17 and 18 July), before full moon (22 and 23 July), after full moon (29 and 30 July) and before new moon (4 and 5 August). The two trap sizes and the two bait pot designs were compared. All traps had straight funnels. Four traps of each size were used with each of the bait pot designs being used in two traps of each size. For each soak, four traps (one of each combination of trap size and bait pot type) were allocated to a different randomly selected 300 m section, with two sets of four traps being used on each day. Within each section traps were placed in a different, randomly selected order. The factors of "trap size," "bait pot design," "lunar period" and "day (within lunar period)" were compared by mixedmodel analysis of variance. The variates analysed were the total numbers of fish and the numbers of each of the four taxa trapped most commonly, A. australis. A. berda, L. russelli and Epinephelus spp. Due to marked non-normality of the data for all variates, caused by zero counts, the means of the catches from the two soaks of each trap on each day were used in analyses. This procedure resulted in data markedly more normal than the original data. The amount of bait remaining in the bait pots at the conclusion of each soak (estimated to the nearest 10%) was also noted. Throughout the study homogeneity of variance was tested using Cochran's test (Winer, 1971) before earrying out analyses of variance. All variables except numbers of Epinephelus spp., required transformation by In(x + I) to produce homogeneity of variance. The data for numbers of Epinephelus spp. were also transformed by In(x + I), however, as these data were markedly non-normal and this transformation tended to normalise the data. For all analyses of variance both transformed and untransformcd data produccd the same results in terms of significance, so while results of analyses on transformed data are tabulated, raw data have been used in compilation of all associated figures to facilitate biologically realistic interpretations. After all analyses of variance a posteriori comparisons of means using Ryan's Q-test (Day and Quinn, 1989) was conducted on significant factors having three or more levels. RESULTS

During the two parts of the study 1,741 fish of 22 species were trapped (Table I). Catches were, however, dominated by a few species. Three species L. russelli, A. berda and A. australis, contributed 31.4%, 30.4% and 18.7% respectively, while four other species E. coioides, E. malabaricus. Arothron manilensis and Monodactylus argenteus contributed between 2% and 10% of fishes trapped. The other 15 species contributed less than I % each, being represented by only 1 or a few individuals. Section fa: 2-h Soaks.-Table 2 summarizes analyses of variance comparing different mesh sizes and funnel designs in 2-h daytime soaks, at three different tidal states, over the course of one lunar cycle. The trap catches did not differ significantly between days for any of the variables studied. The factor of tidal state

478 Table I. study

BULLETIN OF MARINE SCIENCE, VOL. 56. NO.2, 1995

Total numbers

and taxonomic

composition

of fish trap catches

during the Alligator

Size range (mm)

Ariidae Batrachoididae Carangidae Centropomidae Gobiidae Haemulidae Leiognathidae Terapontidae Lutjanidae

Monodactylidae Serranidae Siganidae Sparidae Tetraodontidae

Total

Arius argyropleuron Halophryne diemensis Cararu ignobilis Psammoperca waigiensis Butis butis Pomadasys Iwalwn Cazza minuta Leiognarhus equulus Terapon rheraps Lutjanus argentimaculatus Lutjanus johnjj Lutjanus russelli Monodactylus argenteus Epinephelus coioides Epinephelus malabaricus Siganus lineatus Acanthopagrus australis Acanthopagrus berda Arothron manilensis Arothron reticularis Lagocephalus lunaris Chelonodon patoca

263-308 250-290 137 261 85 63-301 43 35-47 160-189 195-251 127 65-200 62-106 168-456 220-610 68-127 77-213 55-193 141-242 176 251-254 40-199

Number

4 10 I 1 I 14 1 3 2 2 1 546 47 141 53 2 325 529 52 1 2 3

Creek

Percentage of catch

0,2% 0.6% 0.1% 0.1% 0.1% 0,8% 0.1% 0.2% 0.1% 0.1% 0,1% 31.4% 2,7% 8.1% 3.0% 0.1% 18.7% 30.4% 3.0% 0.1% 0,1% 0,2%

1,741

was significant for total fish, A. australis and L. russelli. Both low and mid tide produced significantly higher catch rates than high tide, for total fish (mean ± SE - low 3.76 ± 0.45; mid 3.70 ± 0.45; high 2.13 ± 0.25) and for L. russelli (mean ± SE - low 0.94 ± 0.25; mid 0.84 ± 0.12; high 0.33 ± 0.10), while for A. australis (mean ± SE - low 1.00 ± 0.19; mid 1.00 ± 0.25; high 0.41 ± 0.09) low tide p:roduced significantly higher catches than high tide but catches at mid tide could not be separated from the other two tidal states. For A. australis the significant maiIlleffect of tidal state was further complicated by an interaction with entrance funnel type. However, a posteriori testing was unable to separate any of the means. Graphical comparison suggests that the interaction detected by the analysis of variance was probably a change in the ranking of the two funnel types at mid tide compared to the other tidal states (Fig. 1). Small mesh traps produced significantly higher catches for total fish (mean ± SE - small 4.09 ± 0.38; large 2.30 ± 0.23), A. berda and L. russelli, while no difference was found for A. australis. Although for A. berda the effect was complicated by an interaction with day and funnel, no difference was detected by a posteriori testing and no clear pattern of differences was apparent. For L. russelli the strong main effect of mesh size was complicated by an interaction with funnel type (Fig. 2). A poslteriori comparison of means showed that for both funnel types small mesh produced higher catches than large mesh. However, while horse-head funnels produced higher catches than straight funnels in large mesh no significant differences between funnel designs could be found for small mesh traps. Funnel design was not independently significant for any of the variates. Comparison of Size Structures in Two Mesh Sizes.-There was no significant difference between the size structures of Epinephelus spp. caught in 12.5 mm and 42 mm mesh traps (K-S statistic 0.04, P = 1.0000). There were however, highly

479

SHEA YES: ASH TRAP PERFORMANCE

Table 2. Analyses of variance results for the effects of trap cntrance-funnel design, mesh size, tidal state and different days during one lunar cycle, on total numbers of fish and numbers of three estuarine fish species in fish trap catches using 2-h daytime trap soaks (all data have been transformed by z = In(x

+

Source

I)

of

variation

Day (D) Tide (T) Mesh (M) Funnel (F) DXT DXM DXF TXM TXF M XF DXTXM DXTXF DXMXF TXMXF DxTxMXF Residual

df

9 2 I

1 22 9 9 2 2 1 18 18 9 2 18 120

Total fish

A. australis

A. berda

F

F

F

1.79 7.12** 15.35*** 0.77 0.84 0.80 0.81 0.66 2.72 1.53 0.83 0.76 1.87 0.59 0.83

1.73 4.28* 0.12 0.23 1.15 0.65 1.70 0.96 3.20* 1.21 1.l0 1.02 1.87 0.03 0.71

1.61 2.04 6.05* 0.46 1.20 1.26 1.14 0.02 1.08 3.56 0.75 0.93 2.78** 0.33 1.06

L russelli F

0.97 3.62* 31.78*** 0.01 0.74 1.72 0.65 2.36 0.63 6.02* 0.86 0.46 0.61 0.83 0.49

" Denotes significance at a level of P ;;;0.05 . •• Denotes

significance

at a level of P = 0.0 I. P ;;; 0.001.

*** Denotes significance at a level of

significant differences in size structure of both of the sparids, A. berda (K-S statistic 0.82, P = 0.0000) and A. australis (K-S statistic 0.74, P = 0.0000) between the two meshes. Both species show similar patterns with 77.8% of the catch of A. berda in the small mesh traps being individuals between 70 mm and 110 mm while 83.1% of the catch in large mesh traps was between 110 mm and 160 mm (Fig. 3a). For A. australis 76.8% of the individuals in small mesh traps were between 70 mm and 120 mm and 78.2% of individuals in large mesh traps were between 110 mm and 170 mm. For L. russelli clear difference between the size structures from the two meshes (Fig. 3b) obviated the need for statistical comparisons. In the 12.5 mm mesh 72.2% of the 291 L. russelli individuals trapped were below 140 mm, smaller than any of the L. russelli captured in the 42 mm mesh. However, in contrast to the situations of the two sparids, the L. russelli upper size range limit was similar for both mesh sizes. Section Ib: Comparison of Short (2-h) and Long (11f2-d)Trap Soaks.-Table 3 displays analyses of variance results comparing the effects of short (2-h) daytime and long (11h-d)trap soaks, different trapping days, mesh size and entrance funnel design. The effect of soak time was significant for all variables with the exception of L. russelli. For Epinephelus spp. catches were greater in long soaks (mean ± SE - long 0.52 ± 0.09; short 0.22 ± 0.07). For all the other variables, however, the short daytime soaks produced higher catch rates than the long soaks. This latter pattern was also apparent for L. russelli, for which the difference was very close to being significant (P = 0.0505). For total fish and for A. berda the effect of soak time was modified by interactions with other factors. For the total numbers of fish, soak time interacted strongly with funnel type, with the only difference detected being a greater catch during short soaks than during long soaks in traps with straight funnels (Fig. 4). For A. berda, soak time interacted with mesh size (Fig. 5). For total numbers of fish an interaction of a very similar form occurred. For both these variables, a posteriori testing revealed that small mesh produced

480

BULLETIN OF MARINE SCIENCE, VOL. 56, NO.2, 1995

Nurn bers of flsh/t rap soak 2,5

2

1.5

0,5

o

HIgh

Low

MId

Tidal state

I

D· Straight funnel

---+-

Horse-head

funnel

I

Figure I, Effect of the interaction of tidal state and entrance funnel design on the numbers of A. australis in trap catches during 2-h trap soaks (40 trap hauls/treatment), Data presented are means ± standard errors.

higher catch rates than large only during short soaks and that there was no apparent difference between soaks for large mesh traps, Mesh size had a significant effect on catch rates for both total numbers of fish and for L. russelli. In both cases small mesh produced higher catches than large mesh (L. russelli mean ± SE - small 0.98 ± 0.20; large 0.10 ± 0.03). For total numbers of fish this effect was confined to short soaks, as described for the soak time/mesh size interaction, above. Horse-head funnels produced greater catches of A. berda than straight funnels for both soak times (mean ± SE - horse-head 1.19 ± 0.19; straight 0.70 ::±: 0.11). Mesh size and funnel type interacted for A. australis (Fig. 6). No difference was found between funnel types for small mesh traps but straight funnels produced higher catches than horse-head funnels for large mesh traps. Furthermore, large mesh traps produced significantly higher

SHEAVES:

Number

FISH

of fish/trap

TRAP

481

PERFORMANCE

soak

2

1.5

0.5

o

Stralgh1

Horse-head

Funnel design

I

---*- Small

mesh

~

Large mesh

Figure 2. Effect of the interaction of mesh size and entrance funnel design on the numbers of L. russelli in trap catches during 2-h trap soaks (60 trap hauls/treatment). Data presented are means ± standard errors.

catches than small mesh in straight funnelled traps but significantly lower in traps with horse-head funnels. No differences were detected between days for any of the variables. For the four major taxa trapped during the study (A. australis, A. herda, L. russelli and Epinephelus spp.) the proportions in catches from 2-h and J1h-dtrap soaks were compared and found to differ significantly (X24df = 24.48, P = 0.0001) (Fig. 7). To a large extent this difference was probably related to the greatly increased importance of Epinephelus spp. in Ph-d trap soaks, due to the increased catches of this genus in the longer trap soaks. Section 2.-Analyses

of variance showed no effect of trap size on the total catch of fish or on the catches of A. australis, A. herda, or Epinephelus spp. There was however, a significant difference in the catches of L. russelli between the two trap sizes (F = 9.0998; df = 1,4; P = 0.0393), with mean catch rates in the larger traps being more than double those in the smaller traps (mean ± SE - small 0.86 ± 0.27; large 1.97 ± 0.34). No significant effect of lunar phase or bait pot

482

BULLETIN OF MARINE SCIENCE, VOL. 56, NO.2, 1995

Frequency

80

(0) 60

40

20

o 55

155

130

105

80

180

205

Frequency 50

40 30 20 10

o 55

80

105

130

155

180

205

Size class mid-point Mesh [

--f}-

size

12.5mm

-B-

42mm

Figure 3. Comparisons of the size structures of a) A. berda (N = 396), b) L russelli (N = 269), between small (12.5 mm) and large (42 mm) trap mesh sizes. All data are absolute frequencies with the exception of L russelli for 42 mm mesh where absolute frequency X5 are presented.

type and no significant interactions were detected for any of the variables. In each case the catch between days within a particular part of the lunar cycle varied greatly with no clear pattern of difference between parts of the lunar cycle. There were considerable differences in the amounts of bait remaining in the two bait pot designs. when the traps were lifted. Of the bait pots with fewer but larger holes (design A) 11% had no bait remaining and 33% had less than 20% of the original bait remaining. Of the bait pots with smaller but more numerous

483

SHEA YES: FISH TRAP PERFORMANCE

Table 3. Analyses of variance results comparing the mean catch of 2-h daytime trap soaks to 1Y..-d trap soaks for mesh size and trap funnel design over 8 days. Total numbers of fish and numbers of the four taxa of fishes trapped most commonly are compared. For each variable, analysis was conducted on data traformed by z = In(x + I).

Source of

variation

Soak (S) Day (D) Mesh (M) Funnel (F) SXO S X M S X F DXM DXF MXF SXDXM SXDXF SXMXF DxMxF SXDXMxF Residual

df

I 7 I I 7 I I 7 7 I 7 7 I 7 7 64

Total fish F

5.02* 0.75 6.08* 0.00 0.42 4.02* 7.62** 0.74 1.10 1.34 0.98 0.84 0.85 0.88 1.20

A.

australis F

4.57* 1.I3 0.09 2.24 0.70 0.40 0.18 0.29 1.11 9.59** 0.77 0.52 0.00 1.98 1.76

Epinephelus spp. F

L russell;

F

12.06*** 1.39 1.95 5.03* 0.09 6.93* 3.61 1.81 0.68 1.25 0.40 0.73 0.63 1.40 1.28

4.78* 0.57 0.00 0.09 0.60 0.15 0.00 0.50 0.44 0.33 0.70 0.55 1.17 0.41 0.55

3.97 1.04 27.12*** 0.68 0.88 2.96 3.11 1.14 1.33 0.01 1.24 0.70 2.45 0.66 0.53

A. berda

F

= 0.05. = 0.01 . significance al a level of P = 0.00 I.

.•.Denotes significance at a level of P

** Denotes significance at a level of P

•*. Denotes

holes (design B) none had their bait exhausted and only 9% had less than 20% of bait remaining. DISCUSSION

a majority of the variables tested, there was a clear difference in trap catch rates between different tidal states during short trap soaks. The general pattern was for lowest catches to be made at high tide. The explanation for this pattern is, however, much less clear. In part the observed effect probably resulted from a diminished ability, at high tide, for the researcher to accurately place traps close to the areas of high structural heterogeneity tMgeted. Such areas have been found to produce significantly higher catches than similar areas lacking structural complexity (Sheaves, 1992). Thus traps placed further from such structures would be expected to produce lower catch rates. A number of studies have noted reduced catches of particular species in traps set at increasing distances from reefs (High and Ellis, 1973; Luckhurst and Ward, 1987). On the other hand, it may be that fish exhibit different behaviours between higher and lower tidal levels, which could contribute to the observed effect. For example fish moving into the mangrove forest at high tide (Blaber et at, 1985; Blaber, 1986; Robertson and Duke, 1990) may become unavailable for capture. Influence of Tidal State on Trap Catches.-For

with the smaller mesh size produced the highest catch rate for total fishes during short (2-h) trap soaks. This result is a clear reflection of similar patterns in catches of, A. berda and L. russelli, the two species trapped most commonly. Higher catches in smaller mesh traps is to be expected if individuals of sizes too small to be retained in the larger mesh are present in the population being sampled. It would be expected that smaller individuals would generally be more common given a mortality induced decline of the numbers of fish in particular age groups, over time. This result, taken in

Influence of Mesh Size on Trap Catches.-Traps

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BULLETIN OF MARINE SCIENCE, VOL. 56, NO.2, 1995

Number of fish/trap

soak

5

4

3

2

o Straight

Horse head

Entrance funnel design

I

L-a-

Soak time 2 hour

~

1 1/2

day

Figure 4. Effect of the interaction between length of soak and entrance funnel design on the total number of fish in trap catches (32 trap hauls/treatment), Data presented are means ± standard errors.

isolation, suggests that for most purposes, in the estuary studied, the use of smaller mesh traps is probably desirable as they produce higher catch rates, However, the overall superiority of small mesh traps over large is called into question by the comparisons of the size structures of catches of A. berda and A. australis, For both of these species, the small mesh traps captured very low numbers of individuals in the larger size classes. This occurred despite both small and large mesh entrance funnels being of the same size and traps of each mesh size being spatially interspersed. It seems that larger individuals tended to be captured less efficiently in smaller mesh traps than in larger mesh traps. Marked differences in size structure between mesh sizes were found for L. russelli also. However, in contrast to the situation for A. berda and A. australis, the upper limit of both size distributions was similar. This is the type of pattern expected if fish of different sizes displayed similar responses to the traps. This suggests that small mesh traps would probably be superior for sampling L. russelli in this habitat. However, the sudden truncation of both distributions at around 200 mm still begs the question "Is it that larger fish are absent or does L. russelli show a sudden change in response to traps at this size?"

485

SHEAVES: FISH TRAP PERFORMANCE

Numbers of fish/trap

soak

2

1.5

0.5

o

2 hour

1 1/2 day

Soak time

Mesh size

-e-

Small (12.5mm)

~

Lorge (42mm)

Figure 5. Effect of the interaction between length of soak and mesh size on thc catch of A. berda in traps (32 trap hauls/treatment). Data presented are means ± standard errors.

The two mesh sizes compared differed in design (12.5 mm square and 42 mm hexagonal) as well as in size. The comparison is therefore subject to some confoundmenl. As a result the conclusions of the mesh size comparisons should be considered with this in mind. Influence of Entrance Funnel Design on Trap Catches.-Higher catches of fish in straight funnels during short soaks than during long soaks contrasted with catches in horse-head funnels which did not differ significantly between soak times (Fig. 4). This can probably be attributed to different rates of escapement from the two funnel designs. As the attractiveness of the trap bait has been found to decrease quite rapidly (Whitelaw et aI., 1991) the number of fish attempting to leave the trap relative to those entering probably increases over time. The effect of horse-head funnels in reducing escapement (Munro, 1974; Luckhurst and Ward, 1987) would tend to retain fish in traps with these funnels, while the fish in traps with straight funnels would probably find escapement more easy and thus their numbers would tend to decrease over longer soak times.

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BULLETIN OF MARINE SCIENCE. VOL. 56. NO.2, 1995

Numbers of fish/trap

soak

1.4

1.2

0.8

0.6

0.4

0.2

o Smoll mesh

Lorge mesh

Mesh size

Funnel design -l3- Straight

-+-

Horse-head

Figure 6. Effect of the interaction between mesh size and entrance funnel design on the number of A. australis in trap catches over both short and long soak times (32 trap hauls/treatment). Data presented are means ± standard errors.

Influence of the Length of Trap Soak on Catches.- The comparison of the two soak times showed that, for most species, greater numbers of fish were taken in the short, 2-h, soaks than in the longer, 1~-d, soaks, While only two soak times were investigated and no short soaks were carried out at night, the results of that comparison suggest that for a number of species short soaks can produce catches at least as high as longer soaks. Higher catches in short soak times was also the general trend on Australia's North West Shelf (Whitelaw et al., 1991). In many situations there are clear advantages to using quite short soak times, if this does not reduce catch per trap. Not only can many more replicate soaks be made per unit time, but short soaks provide opportunity to conduct investigations over short temporal scales. In this study, for example, short soaks allowed the detection of differences due to tidal state. Comparison of Trap Sizes.--In other studies where trap sizes have been compared, large traps have generally been found to produce higher catch rates (Munro, 1974; Crossland, 1976). In this study, using traps of twice the volume increased

SHEA YES: FISH TRAP

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PERFORMANCE

Percentage of catch

100%

75%

50%

25%

0%

1 1/2

2 hour soak

D

A. australis

_

[pinephelus

sP. ~

A. berda

D

day soak

L russelli

Other

Figure 7. Comparison of the proportional composition of catches in short (2-h) and long (Ph-d) trap soak times (64 trap hauls/treatment).

the trap catch rate of L. russel/i. Clearly, if L. russel/i was the species of interest there would be an advantage in using the larger traps. However, the larger traps did not produce higher catch rates for the total number of fish or for any of the other species considered. This suggests that, for many species, useful increases in catch rates do not necessarily accrue from trap size increases. Where catches are not greatly improved by using larger traps, the increased handling and transport ease of small traps (allowing, for example, more replicate traps) may become important factors influencing trap size selection. Comparisons of Bait Pot Designs.-While both bait pot designs produced similar catch rates for all variables, considerable differences were found in amount of bait remaining at the conclusion of the trap soak. This was probably because the diameter of the smaller holes was less than the diameter of most pilchards, while the diameter of the larger holes was greater than that of the pilchards. Thus fish could more easily pull the pilchard bait out of containers with the larger holes. Bait retention associated with a particular bait pot design has important implications. The presence of bait in a trap increases the rate at which fish enter (Munro, 1974; Whitelaw et aI., 1991). If the bait supply in a particular trap is

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BULLETIN OF MARINE SCIENCE, VOL. 56, NO.2, 1995

exhausted (or even nearly so) it is unclear how the catch in this trap relates to the catch in other traps which still have bait remaining. Is the comparison really between baited traps or is the comparison effectively between unbaited and baited traps? While it is important to consider bait retention in the design of bait pots, this must be balanced against higher rates of trap entry resulting from a large plume of bait escaping from the trap (Whitelaw et aI., 1991). CONCLUSIONS

The differing responses of particular taxa to different soak times, entrance funnels, trap sizes and mesh sizes noted in this and other studies suggests that all aspects of trap design and management need to be considered when designing fish trapping programs, Furthermore, the difference in catch rates during different tidal states emphasises the need to standardise trapping with respect to environmental variables, Clearly, there is a need to tailor the design of fish traps and of trapping programs to target particular species of interest and to meet the demands of particular sampling schemes. LITERATURE

CITED

Blaber, S. J. M. 1986. Feeding selectivity of a guild of piscivorous fish in mangrove areas of northwest Australia. Aust. J. Mar. Fresh, Res. 37: 337-345. ---, J. W. Young and M. C. Dunning. 1985. Community structure and zoogeographic affinities of the coastal fishes of the Dampier region of north-western Australia. Aust. J. Mar. Fresh. Res. 36: 247-266. Bohnsack, J, A., D, L. Sutherland, D. E. Harper, D. B. McClellan, M. W. Hulsbeck and C. M. Holt. 1990, The effects of fish trap mesh size on reef fish catch off southeastern Florida. Mar, Fish. Rev. 51(2): 14-30. Crossland, J. J. 1976. Fish trapping experiments in northern New Zealand waters. N. Z. J. Mar. Fresh. Res. 10(3): 511-516. Dalzell, P. and J, W. Aini. 1987. Preliminary results of fish trials with arrowhead fish traps in Papua New Guinea. So. Padf. Comm. Fish. News. 41: 34-40. Day, R. W. and G, P. Quinn. 1989. Comparisons of treatments after an analysis of variance in ecology. Eco. Monog. 59: 433-463. High, W. L. and T. E. Ellis. 1973. Underwater observations of fish behavior in traps. Helgolander Wiss. Meeresunters. 24: 341-347. Kulbicki, M. and G. Man-Tham. 1987. Essais de peche au casier a poissons dans Ie lagon de Nouvelle Caledonie. Rapp. Sci. Tech. BioI. Mar. 47: 22 pp. Luckhurst, B. and J. Ward. 1987. Behavioural dynamics of coral reef fishes in Antillean fish traps in Bermuda. Proc. Gulf Carib. Fish. Inst. 38: 528-546. Munro, J. L. 1974. The mode of operation of Antillean fish traps and the relationships between ingress, escapement. catch and soak. J. Cons. 35: 337-350. ---. 1986. Construction details of Antillean fish traps. ICLARM publication. ---, P. H. Reeson and V. C. Gaunt. 1971. Dynamic factors effecting the performance of Antillean fish traps. Proc. Gulf Carib. Fish. Inst. 23: 184-194. Powles, H. and C. A. Barans. 1980. Groundfish monitoring in sponge-coral areas off the southeastern United States. Mar. Fish. Rev. May, 1980: 21-35. Prabhu, M. S. 1954. The perch-fishery by special traps in the area around Mandapam in the Gulf of Mannar and Palk Bay. Ind, J. Fish, 1(1 and 2): 94-129. Recksiek, C. W., R. S. Appeldoorn and R. G. Turingan. 1991. Studies of fish traps as stock assessment devices on a shallow reef in south-western Puerto Rico. Fish. Res. 10: 177-197. Robertson, A. I. and N. C. Duke. 1990. Mangrove fish-communities in tropical Queensland, Australia: spatial and temporal patterns of densities, biomass and community structure. Mar. BioI. 104: 369379. Sheaves, M. J. 1992. Patterns of distribution and abundance of fishes in contra.,ting habitats in a mangrove-lined tropical estuary as determined by fish traps. Aust. J. Mar, Fresh. Res. 43(6): 14611479. Stevenson, D. K. and P. Stuart-Sharkey. 1980. Performance of wire fish traps on the western coast of Puerto Rico. Proc. Gulf Carib. Fish. Inst. 32: 173-193. Ward, J. 1988. Mesh size selection in antillean arrowhead fish traps. FAG Fish. Rep. 389: 455-467,

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Whitelaw, A. W., K. J. Sainsbury, G. J. Dews and R. A. Campbell. 1991. Catching characteristics of four fish-trap types on the North West Shelf of Australia. Aust. J. Mar. Fresh. Res. 42: 369-382. Winer, B. J. 1971. Statistical principles in experimental design. McGraw-Hili, Sydney. 907 pp. Wolf, R. S. and G. R. Chislett. 1974. Trap fishing explorations for snapper and related species in the Caribbean and adjacent waters. Mar. Fish. Rev. 36(9): 49-61. DATE ACCEPTED:

October

ADDRESS: Department Qld. 48/ J, Australia.

8, 1993.

of Marine Biology, Jame.f Cook University of North Queensland,

Townsville