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Effect of trap design and soak time on catches of the British Columbia prawn (Pandalus platyceros)
Fisheries Research, 6 (1987) 69-79
69
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Effect of Trap D e s i g n and S o a k T i m e on Catches of the British Columbia P r a w n ( P a n d a l u s
platyceros ) J.A, BOUTILLIER and N.A. SLOAN
Department o/Fisheries and Oceans, Biological Sciences Branch, Pacific Biological Station, Nanaimo, British Columbia V9R 5K6 (Canada) (Accepted for publication November 11, 1986)
ABSTRACT Boutillier, J.A. and Sloan, N.A., 1987. Effect of trap design and soak time on catches of the British Columbia prawn (Pandalus platyceros). Fish. Res., 6: 69-79. In 1985 three experiments, concluding a series began in 1977, were completed for the evaluation of variables considered important for defining effective trap fishing effort for the prawn, PandaIus platyceros, in British Columbia. The effect on catch of trap volume, number of tunnels, tunnel length and tunnel type were examined for different soak times. Trap volume and tunnel type significantly influenced catches, whereas the number of tunnels and tunnel length did not. Soak time alone had a significant influence in the trap volume experiment or in interaction in the trap volume or tunnel length experiments. These variables are discussed in relation to developing a standardization algorithm permitting improved interpretation of catch and effort data for prawn stock assessment.
INTRODUCTION
From 1976 through 1985, fishing effort in the British Columbia prawn, Pandalus platyceros Brandt, trap fishery has increased dramatically, from 48 to 241 vessels. The fishery is scattered over a wide range of discrete nearshore areas, especially inlets. Harvest methods, as to depth, soak time and baiting, vary greatly. There are no gear restrictions, with at least 13 general trap categories and > 30 trap designs being used (Boutillier, 1985). We report on three experiments which end a series (Boutillier, 1985, 1986) identifying important variables for defining effective fishing effort in this fishery. Effective fishing effort is defined by Ricker (1975) as the adjusted measurement of effort such that each increase in the adjusted unit causes a proportional increase in instantaneous rate of fishing. Estimating effective fishing effort is important for developing a fisheries catch and effort data base 0165-7836/87/$03.50
© 1987 Elsevier Science Publishers B.V.
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Fig. 1. Alberni Inlet, west coast of Vancouver Island, British Columbia. Star Point (SP), site of number of tunnels experiment; East Spencer (ES), site of tunnel length experiment; Coleman Creek (CC), site of trap volume experiment. for stock assessment in the P. platyceros trap fishery (Boutillier, 1986). With an aim to calibrating effort, we examined P. platyceros catches according to trap volume, number of tunnels and tunnel length at different soak times. METHODS AND RESULTS
Study sites, trap deployment and statistical methods Three experiments were completed in Alberni Inlet (Fig. 1 ) during the summer of 1985. Alberni Inlet is a deep ( > 300 m ) , steep-walled fjord on the west coast of Vancouver Island and has been one of the major commercial prawn producing areas in British Columbia. The experimental sites were chosen from areas yielding commercial catch rates of P. platyceros, as determined by a series of pre-study sets. In all experiments, trap types were set in random sequence, 10 m apart on a
71 TABLE I Characteristics of traps used in the trap volume experiment Trap characteristics
Tunnel characteristics
Size Dimensions( c m ) VolumeNo. of Length Outside opening Insideopening class (1) tunnels (cm) Top Bottom Side Shape Area Shape Area diam. diam. height (cm2)~ ( cm2) S M L
46X 56× 63 X 76 X 81× 97×
24 30 35
48 110 210
3 3 3
14 18 20
Rectangular299 Rectangular495 Rectangular798
Round 57 Round 57 Round 57
All traps were of the conical nesting type, composedof a stainless steel frame of two hoops of differentdiameters connectedby six cross-members.Trap mesh was black, knotted nylon 'herring bunt' web 2.4-cm stretch mesh, knot-to-knot. Tunnels had larger outside openings tapering to smaller inside openings. groundline deployed at 80-100 m depth at the study sites. The order in which differing soak times (24-96 h) were fished within each experiment was randomized. The industry standard is 24 h (Boutillier, unpublished observation ). All traps were baited with a 100-g can of sardines packed in edible oil. Each bait can was performed with four holes of 6.0 mm in diameter. Traps were freshly baited each time they were set. Captured prawns were counted, their carapace length (posterior orbital rim to median dorsal carapace edge) measured and retained. A two-way ANOVA was used to analyze normally distributed data and a Kurskal-Wallis analysis was used to analyze data which were not normally distributed. All analyses were performed with the B M D P statistical package (Dixon, 1985). The data for each experiment were tested for normality and equal variances using diagnostic plots of the relationships between cell means and cell standard deviations and testing for deviation from a normal distribution of the skewness and kurtosis. Cell means are mean catch rates of a group of traps based on values of their independent variables, e.g. all similar sized conical nesting traps soaked for 24 h are in a cell group. The regression of the log of cell standard deviation on the log of the cell mean was used to determine what, if any, transformations could be used to make the data variances homoscedastic.
Trap volume experiment To test whether trap volume influences prawn catches, we used three sizes of conical nesting traps whose specifications are listed in Table I. Soak durations were 24, 48, 72 and 96 h. Eight traps of each type were set three times for
72 50
~ O'-'-~ ~
S M A L L TRAPS MEDIUM T R A P S LARGE TRAPS
40 ¸
==
+~ 20 _..=
,
,
24
,
,
48
,
72
,
,
96
SOAK TIME (h}
Fig. 2. Mean number of P. platyceros caught by three sizes (volumes) of conical nesting traps at four soak times.
each soak duration. T r e a t m e n t effects (size and soak time) were analyzed by ANOVA of square root-transformed number of prawns caught. Levels of treatments were compared with a Tukey multiple comparison tests (Zar, 1984 ). Figure 2 illustrates the catch of prawns over time. ANOVA results reveal that soak time, trap volume and their interaction all significantly influenced catches (Table II). An a posteriori Tukey test showed that there was no significant difference between catches of any trap size at either 24- or 48-h soaks (P > 0.05 ). Medium and large traps, however, caught significantly more prawns at 72- and 96-h soaks than small traps at any soak time ( P < 0.05). Also, the largest traps at 72- and 96-h soaks caught significantly more prawns than either medium or large traps at 48-h soaks ( P < 0.05). TABLE II ANOVA results using square root of the number of Pandalus platyceros caught as the dependent variable against soak time and trap volume in conical nesting traps Source
df
Mean square
F
Soak time Trap volume Soak/trap interaction Error
3 2 6 276
16.66828 64.59813 8.31914 1.53108
10.89"* 42.19"* 5.43**
**P < 0.01.
73
Number of tunnels experiment To evaluate whether number of tunnels influences prawn catches, we used three different trap designs in which the number of tunnels varied between two and four. Trap specifications are listed in Table III. Soak durations were 24 and 48 h. Five traps of each type were set seven times for each soak duration. Data were analyzed separately for each trap type by a two-factor Kruskal-Wallis test (Zar, 1984). There was high variance among all treatment replicates (Fig. 3) and the Kruskal-Wallis test showed no significant effect of either the number of tunnels or soak time on prawn catches (Table IV).
Tunnel length experiment The effect of tunnel length on prawn catches was examined using four tunnel designs in similar sized conical nesting traps whose specifications are listed in Table V. Soak durations were 24, 48, 72 and 96 h. Eight traps of each type were set three times at each soak duration. Data were analyzed by ANOVA of the fourth root-transformed number of prawns caught. Tukey tests were used to identify levels of variables (tunnel length; soak time ) significantly influencing catches of prawns. The prawn catches illustrated in Fig. 4 show similar catches over time in medium- and long-tunnelled traps, a tendency towards prawn escapement with increasing soak time in short-tunnelled traps and an increasing catch in inserttunnelled traps over time. ANOVA results demonstrate that tunnel length and soak time/tunnel length interaction significantly influenced prawn catches (Table VI). An a posteriori Tukey test revealed that only the insert tunnel traps for 24-h soaks had significantly lower catches than short tunnel traps at 24-h soaks, medium tunnel traps at all soak times and long tunnel traps at 72and 96-h soaks. Moreover, insert tunnel traps at 48-h soaks yielded significantly fewer prawns than medium tunnel traps at 24-h soaks. Best performance came from medium length tunnelled traps at 24-h soaks, followed by long tunnelled traps at 72-h soaks. DISCUSSION Larger traps caught more prawns than smaller traps of the same design, as do larger traps generally for prawns (Dahlstrom, 1963; Ronholt, 1974; Boutillier, 1985) and crabs (Miller, 1979). We found no significant difference between prawn catches in traps soaked 24 or 48 h and only after 72-96 h did the small traps demonstrate saturation, while larger traps continued to significantly increase their catches. Saturation level is the point at which there is no further entry or the rate of entry equals escapement (Miller, 1979). Antago-
TABLE III
Trap characteristics
Bottom
Side
Dimensions (cm) Top 30
24
24
76a X
45 X
60 a X
63~X
60 a ×
45 X
No. of tunnels
18 18 18
Length (cm)
Rectangular Rectangular Rectangular
Rectangular Rectangular Rectangular
Shape
253 253
912 912 912
705 495 705
(cm 2)
Oval Oval
Round Round Round
Round Round Round
Shape
36 36
24 24 24
57 57 57
(cm 2)
Tunnel characteristics
2 3 4
18 18 18
Rectangular Rectangular
Mesh characteristics
110
Black, knotted nylon 2.4-cm stretch mesh, knot-to-knot
2 3 4
8 8
Inside opening
68
Black, knotted nylon 2.4-cm stretch mesh, knot-to-knot
2 4
Outside opening
49
Rigid, galvanized wire m e s h with 2.4 X 1.2-cm sides
Volume (1)
Characteristics of traps used in tunnel number experiment. Tunnels had larger outside openings tapering to smaller inside openings Trap type
Conical nesting
Round 'Pardiac'
Square 'wire mesh'
"Diameter.
2.
75 30
CONICAL NESTING TRAPS
SOAK TIME 0----.0 24h H 48h
25
zo
o_ <
u_ ©
15
15
I0
0..
D z
15
,el
t~
c~ i.u m
ROUND PARDIAC T R A P S
SQUARE WIRE MESH TRAPS
iO
.I-'" .....,
>_.~
5 I 0
0
2
3
4
,
I
0
2
2
4
NUMBER OF TUNNELS PER TRAP
Fig. 3. Mean number of P. platyceros caught by three trap types, with varying number of tunnels, at two soak times. 30"
ac
~ 0----.0
301 "--
SHORT TUNNEL TRAPS INSERTTUNNEL TRAPS
2.
Z~----'%
20"
20
I0-
I0
MEDIUM T U N N E L TRAPS LONG T U N N E L TRAPS
" 5a_
+1
5~
m <•
tI
0 24
48
72
96
24
48
72
96
SOAK TIME (h}
Fig. 4. Mean number of P. platyceros caught by similar sized conical nesting traps with different tunnel lengths at four soak times.
nism between prawns crowded in a trap or between captured prawns and those trying to enter the trap could determine saturation level. With regard to crabs, Miller (1979) and Breen (1985) proposed that saturation was caused by captured individuals intimidating those trying to enter the trap. Increasing prawn trap volume decreases the chance of encounters and increases the saturation level. Volume is an important variable when standardizing a unit of effective fishing effort.
76 TABLE IV Kruskal-Wallis results of the ranks of the number of Pandalusplatyceros caught as the dependent variable against soak time and tunnel number according to trap type: (A) conical nesting; (B) round 'Pardiac'; (C) square wire mesh Source
(A)
(B)
(C)
Kruskal-Wallis test df
Statistic
P
Conical nesting traps Soak time Tunnel number Soak/tunnel interaction
1 2 5
2.16 3.13 8.38
0.1417 0.2095 0.1365
Round 'Pardiac' Soak time Tunnel number Soak/tunnel interaction
1 2 5
2.00 1.03 3.38
0.1574 0.5964 0.6414
Square wire mesh Soak time Tunnel number Soak/tunnel interaction
1 1 3
0.09 0.08 0.41
0.7618 0.7778 0.9373
T h e n u m b e r of t u n n e l s w i t h i n t h e t h r e e t r a p s d e s i g n s u s e d did n o t a f f e c t p r a w n c a t c h e s a t e i t h e r soak. S i m i l a r s a t u r a t i o n levels for a t r a p t y p e w e r e r e a c h e d i r r e s p e c t i v e of t h e n u m b e r of t u n n e l s . W e c o n c l u d e t h a t t h e n u m b e r of t u n n e l s is n o t a n i m p o r t a n t v a r i a b l e for c a l c u l a t i n g t h e e f f i c i e n c y of p r a w n traps. TABLE
V
Characteristics of traps used in the tunnel length experiment Tunnel type
Tunnel characteristics Length
Number
Internal opening
External opening
(cm) Shape
Area
Shape
(cm ~) S M L Inserta
10 18 24 20
3 3 3 3
Rectangular Rectangular Rectangular Round
495 495 495 64
Area
(cm 2) Round Round Round Round
57 57 57 57
"Little-tapering,cylindricaltunnels inserted through a hole cut in the trap's side mesh. All traps were the conical nesting type (volume = 110 l;see Table I), composed of a stainlesssteel frame covered with black, knotted nylon 'herringbunt' web of 2.4 c m stretch mesh, knot-to-knot.
77 TABLE VI ANOVAresults using fourth root of the numberof Pandulus platyceros caughtas the dependent variable against soaktime and tunnel length in similar sizedconicalnestingtraps Source
df
Mean square
F
Soak time Tunnel length Soak/tunnel interaction Error
3 3 9 368
0.29445 0.79458 0.38647 0.15712
1.87 5.06** 2.46**
**P<0.01. Tunnel lengths of 10-24 cm in conical nesting traps did not significantly influence prawn catches. Ronholt (1974) obtained similar results with 23-33cm tunnels for a different trap design. Insert tunnels (20 cm long), however, which had relatively small (Table V) external openings flush with the trap exterior, were in traps which caught significantly fewer prawns than all the other traps with larger external openings. Kessler (1969) found that prawns approaching traps along the substrate tend to be diverted by vertical trap walls, but are more readily led into a trap by gradually tapering tunnel walls from wide external openings. After long soaks, the insert tunnel traps eventually reach a saturation level similar to the other traps. Wide external tunnel openings (tapering to smaller internal openings ) are a more important criteria than tunnel length in determining catch. Morgan (1979) used categories to develop a standardized unit of effective fishing effort, of which two were the fishing power of the gear and the fishing time, i.e. soak time during which the gear is actively fishing. Fishing power variability is caused by two distinguishable sets of factors: the differences between gear type saturation levels and the efficiency with which a gear type reaches its saturation level. In Simpson's (1975) discussion on effort measurement in crustacean trap fisheries, changing trap type altered catch per trap by a factor of two in an area with unchanged stock abundance. We show here variations in saturation levels caused by trap volume. In addition, the fishing power of a single gear type can be related to the design of tunnel entrance, as shown in the length of tunnel experiment. Fishing ( soak ) time during which gear is actively fishing is a variable in all experiments. Soak time had a significant effect on catches, either alone or after interaction with variables such as trap volume and tunnel length. Munro (1974) indicated that the interactions of soak time, gear design, ingress and escapement combine to produce unique asymptotic catch curves in trap fisheries. Munro (1974) and Bennett and Brown (1979) caution that evaluating the influence of soak time on trap catches for stock estimates requires care. For example, Munro (1974) stressed that catch per day soaked can be misleading
78
when dealing with asymptotic catch curves. He suggested an index of 'availability', equal to the mean daily rate of ingress of animals into traps, for a comparable measure of stock density. It is likely that the asymptotic catch was reached before 24 h in some of our experiments (Figs. 2, 3 and 4). Boutillier (1986) found, in some instances, P. platyceros catch per hour soaked declined significantly after 3-6 h. The industry soak standard is ~ 24 h, which may confound interpretation of their catch records since the catch may have reached an asymptotic level. With these experiments and those of Boutillier (1985, 1986) and Boutillier and Sloan (in press ), the groundwork has been laid to develop a data collection system, from either fishermen's log books or test fishing, which can be interpreted to reflect changes in prawn population density rather than just changes in the fishing process. ACKNOWLEDGEMENTS
Our thanks to the skipper and mate of the "M.V. CALIGUS", G. Brown, W. Carolsfeld, W. Harling, S. Head, and D. Heritage for field assistance. G.S. Jamieson, B.D. Smith and S.L. Swarbrick kindly commented on various drafts.
REFERENCES Bennett, D.B. and Brown, C.G., 1979. The problems of pot immersion time in recording and analyzing catch-effort data from a trap fishery. Rapp. P.-V. Reun. Cons. Int. Explor. Mer, 175: 186-189. Boutillier, J.A., 1986. Fishing effort standardization in the British Columbia prawn (Pandalus platyceros) trap fishery. Can. Spec. Publ. Fish. Aquat. Sci., 92: 176-181. Boutillier, J.A., 1985. Important variables in the definition of effective fishing effort for the British Columbia prawn (Pandalus platyceros) trap fishery. J. Shellfish Res., 5: 13-19. Boutillier, J.A. and Sloan, N.A., in press. Trap mesh selectivity in relation to the legal size regulation for prawn (Pandalus platyceros) in British Columbia. J. Shellfish Res., Breen, P.A., 1985. Crab gear selectivity studies in Departure Bay. Can. MS Rep. Fish. Aquat. Sci., 1848: 21-39. Dahlstrom, W., 1963. Prawn shrimp. Calif. Dep. Fish. Game Cruise Dep. 63-A-1. Dixon, W.J. (Editor), 1985. BMDP Statistical Software. (1985 printing). University of California Press, Berkeley, 733 pp. Kessler, D.W., 1969. Test-tank studies of shrimp -pot efficiency. Fish. Ind. Res. (U.S. Fish. Wildl. Serv.), 5: 151-160. Miller, R.J., 1979. Saturation of crab traps: reduced entry and escapement. J. Cons. Int. Explor. Mer, 38: 338-345. Morgan, G.R., 1979. Trap response and the measurement of effort in the fishery for the western rock lobster. Rapp. P.-V. Reun. Cons. Int. Explor. Mer, 175: 197-203. Munro, J.L., 1974. The mode of operation of Antillean fish traps and the relationships between ingress, excapement, catch, and soak. J. Cons. Int. Explor. Met, 35: 337-350. Ricker, W.E., 1975. Computation and interpretation of biological statistics of fish populations. Bull. fish. Res. Board Can., 191:382 pp.
79 Ronholt, L.L., 1974. A study of the relative efficiency of shrimp pots for harvesting the spot shrimp, Pandalus platyceros, in southeastern Alaskan waters. M.Sc. Thesis, University of Washington, Seattle, 101 pp. Simpson, A.C., 1975. Effort measurement in the trap fisheries for crustacea. Rapp. R.-V. Reun. Cons. Int. Explor. Mer, 168: 50-53. Zar, J.H., 1984. Biostatistical Analysis. (Second edition). Prentice-Hall Inc., New Jersey, 718 pp.
69
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Effect of Trap D e s i g n and S o a k T i m e on Catches of the British Columbia P r a w n ( P a n d a l u s
platyceros ) J.A, BOUTILLIER and N.A. SLOAN
Department o/Fisheries and Oceans, Biological Sciences Branch, Pacific Biological Station, Nanaimo, British Columbia V9R 5K6 (Canada) (Accepted for publication November 11, 1986)
ABSTRACT Boutillier, J.A. and Sloan, N.A., 1987. Effect of trap design and soak time on catches of the British Columbia prawn (Pandalus platyceros). Fish. Res., 6: 69-79. In 1985 three experiments, concluding a series began in 1977, were completed for the evaluation of variables considered important for defining effective trap fishing effort for the prawn, PandaIus platyceros, in British Columbia. The effect on catch of trap volume, number of tunnels, tunnel length and tunnel type were examined for different soak times. Trap volume and tunnel type significantly influenced catches, whereas the number of tunnels and tunnel length did not. Soak time alone had a significant influence in the trap volume experiment or in interaction in the trap volume or tunnel length experiments. These variables are discussed in relation to developing a standardization algorithm permitting improved interpretation of catch and effort data for prawn stock assessment.
INTRODUCTION
From 1976 through 1985, fishing effort in the British Columbia prawn, Pandalus platyceros Brandt, trap fishery has increased dramatically, from 48 to 241 vessels. The fishery is scattered over a wide range of discrete nearshore areas, especially inlets. Harvest methods, as to depth, soak time and baiting, vary greatly. There are no gear restrictions, with at least 13 general trap categories and > 30 trap designs being used (Boutillier, 1985). We report on three experiments which end a series (Boutillier, 1985, 1986) identifying important variables for defining effective fishing effort in this fishery. Effective fishing effort is defined by Ricker (1975) as the adjusted measurement of effort such that each increase in the adjusted unit causes a proportional increase in instantaneous rate of fishing. Estimating effective fishing effort is important for developing a fisheries catch and effort data base 0165-7836/87/$03.50
© 1987 Elsevier Science Publishers B.V.
70 I
125"o5
j
'"..'..~:.':.i.':':]
]5,:':'.
:iF"$ .";~
"49°00
• . ::.~:....
..
I
!
i::% 25°oo ~.'..~,,~
' :.~~:~
I::. •
.:,..:~' .t...: \'.'.Jcc ~i,.
.\'9
~...TI,,,
:'~
• ..:1
124o55' ,
7
~':':
.J...::.
S':::: " -~
_
U
~~':'~
~!::i~' . [ ~ ~ . "~;.:i"• [!
,,~
~ .:,:: : :"
o
~
%
a
•
.
l
.:. , . • ..... .
:~: 'i:
.:.:, ::%.!:....:.. .:.. ,:..~-"
125.o'~
1.27° I
I
Fig. 1. Alberni Inlet, west coast of Vancouver Island, British Columbia. Star Point (SP), site of number of tunnels experiment; East Spencer (ES), site of tunnel length experiment; Coleman Creek (CC), site of trap volume experiment. for stock assessment in the P. platyceros trap fishery (Boutillier, 1986). With an aim to calibrating effort, we examined P. platyceros catches according to trap volume, number of tunnels and tunnel length at different soak times. METHODS AND RESULTS
Study sites, trap deployment and statistical methods Three experiments were completed in Alberni Inlet (Fig. 1 ) during the summer of 1985. Alberni Inlet is a deep ( > 300 m ) , steep-walled fjord on the west coast of Vancouver Island and has been one of the major commercial prawn producing areas in British Columbia. The experimental sites were chosen from areas yielding commercial catch rates of P. platyceros, as determined by a series of pre-study sets. In all experiments, trap types were set in random sequence, 10 m apart on a
71 TABLE I Characteristics of traps used in the trap volume experiment Trap characteristics
Tunnel characteristics
Size Dimensions( c m ) VolumeNo. of Length Outside opening Insideopening class (1) tunnels (cm) Top Bottom Side Shape Area Shape Area diam. diam. height (cm2)~ ( cm2) S M L
46X 56× 63 X 76 X 81× 97×
24 30 35
48 110 210
3 3 3
14 18 20
Rectangular299 Rectangular495 Rectangular798
Round 57 Round 57 Round 57
All traps were of the conical nesting type, composedof a stainless steel frame of two hoops of differentdiameters connectedby six cross-members.Trap mesh was black, knotted nylon 'herring bunt' web 2.4-cm stretch mesh, knot-to-knot. Tunnels had larger outside openings tapering to smaller inside openings. groundline deployed at 80-100 m depth at the study sites. The order in which differing soak times (24-96 h) were fished within each experiment was randomized. The industry standard is 24 h (Boutillier, unpublished observation ). All traps were baited with a 100-g can of sardines packed in edible oil. Each bait can was performed with four holes of 6.0 mm in diameter. Traps were freshly baited each time they were set. Captured prawns were counted, their carapace length (posterior orbital rim to median dorsal carapace edge) measured and retained. A two-way ANOVA was used to analyze normally distributed data and a Kurskal-Wallis analysis was used to analyze data which were not normally distributed. All analyses were performed with the B M D P statistical package (Dixon, 1985). The data for each experiment were tested for normality and equal variances using diagnostic plots of the relationships between cell means and cell standard deviations and testing for deviation from a normal distribution of the skewness and kurtosis. Cell means are mean catch rates of a group of traps based on values of their independent variables, e.g. all similar sized conical nesting traps soaked for 24 h are in a cell group. The regression of the log of cell standard deviation on the log of the cell mean was used to determine what, if any, transformations could be used to make the data variances homoscedastic.
Trap volume experiment To test whether trap volume influences prawn catches, we used three sizes of conical nesting traps whose specifications are listed in Table I. Soak durations were 24, 48, 72 and 96 h. Eight traps of each type were set three times for
72 50
~ O'-'-~ ~
S M A L L TRAPS MEDIUM T R A P S LARGE TRAPS
40 ¸
==
+~ 20 _..=
,
,
24
,
,
48
,
72
,
,
96
SOAK TIME (h}
Fig. 2. Mean number of P. platyceros caught by three sizes (volumes) of conical nesting traps at four soak times.
each soak duration. T r e a t m e n t effects (size and soak time) were analyzed by ANOVA of square root-transformed number of prawns caught. Levels of treatments were compared with a Tukey multiple comparison tests (Zar, 1984 ). Figure 2 illustrates the catch of prawns over time. ANOVA results reveal that soak time, trap volume and their interaction all significantly influenced catches (Table II). An a posteriori Tukey test showed that there was no significant difference between catches of any trap size at either 24- or 48-h soaks (P > 0.05 ). Medium and large traps, however, caught significantly more prawns at 72- and 96-h soaks than small traps at any soak time ( P < 0.05). Also, the largest traps at 72- and 96-h soaks caught significantly more prawns than either medium or large traps at 48-h soaks ( P < 0.05). TABLE II ANOVA results using square root of the number of Pandalus platyceros caught as the dependent variable against soak time and trap volume in conical nesting traps Source
df
Mean square
F
Soak time Trap volume Soak/trap interaction Error
3 2 6 276
16.66828 64.59813 8.31914 1.53108
10.89"* 42.19"* 5.43**
**P < 0.01.
73
Number of tunnels experiment To evaluate whether number of tunnels influences prawn catches, we used three different trap designs in which the number of tunnels varied between two and four. Trap specifications are listed in Table III. Soak durations were 24 and 48 h. Five traps of each type were set seven times for each soak duration. Data were analyzed separately for each trap type by a two-factor Kruskal-Wallis test (Zar, 1984). There was high variance among all treatment replicates (Fig. 3) and the Kruskal-Wallis test showed no significant effect of either the number of tunnels or soak time on prawn catches (Table IV).
Tunnel length experiment The effect of tunnel length on prawn catches was examined using four tunnel designs in similar sized conical nesting traps whose specifications are listed in Table V. Soak durations were 24, 48, 72 and 96 h. Eight traps of each type were set three times at each soak duration. Data were analyzed by ANOVA of the fourth root-transformed number of prawns caught. Tukey tests were used to identify levels of variables (tunnel length; soak time ) significantly influencing catches of prawns. The prawn catches illustrated in Fig. 4 show similar catches over time in medium- and long-tunnelled traps, a tendency towards prawn escapement with increasing soak time in short-tunnelled traps and an increasing catch in inserttunnelled traps over time. ANOVA results demonstrate that tunnel length and soak time/tunnel length interaction significantly influenced prawn catches (Table VI). An a posteriori Tukey test revealed that only the insert tunnel traps for 24-h soaks had significantly lower catches than short tunnel traps at 24-h soaks, medium tunnel traps at all soak times and long tunnel traps at 72and 96-h soaks. Moreover, insert tunnel traps at 48-h soaks yielded significantly fewer prawns than medium tunnel traps at 24-h soaks. Best performance came from medium length tunnelled traps at 24-h soaks, followed by long tunnelled traps at 72-h soaks. DISCUSSION Larger traps caught more prawns than smaller traps of the same design, as do larger traps generally for prawns (Dahlstrom, 1963; Ronholt, 1974; Boutillier, 1985) and crabs (Miller, 1979). We found no significant difference between prawn catches in traps soaked 24 or 48 h and only after 72-96 h did the small traps demonstrate saturation, while larger traps continued to significantly increase their catches. Saturation level is the point at which there is no further entry or the rate of entry equals escapement (Miller, 1979). Antago-
TABLE III
Trap characteristics
Bottom
Side
Dimensions (cm) Top 30
24
24
76a X
45 X
60 a X
63~X
60 a ×
45 X
No. of tunnels
18 18 18
Length (cm)
Rectangular Rectangular Rectangular
Rectangular Rectangular Rectangular
Shape
253 253
912 912 912
705 495 705
(cm 2)
Oval Oval
Round Round Round
Round Round Round
Shape
36 36
24 24 24
57 57 57
(cm 2)
Tunnel characteristics
2 3 4
18 18 18
Rectangular Rectangular
Mesh characteristics
110
Black, knotted nylon 2.4-cm stretch mesh, knot-to-knot
2 3 4
8 8
Inside opening
68
Black, knotted nylon 2.4-cm stretch mesh, knot-to-knot
2 4
Outside opening
49
Rigid, galvanized wire m e s h with 2.4 X 1.2-cm sides
Volume (1)
Characteristics of traps used in tunnel number experiment. Tunnels had larger outside openings tapering to smaller inside openings Trap type
Conical nesting
Round 'Pardiac'
Square 'wire mesh'
"Diameter.
2.
75 30
CONICAL NESTING TRAPS
SOAK TIME 0----.0 24h H 48h
25
zo
o_ <
u_ ©
15
15
I0
0..
D z
15
,el
t~
c~ i.u m
ROUND PARDIAC T R A P S
SQUARE WIRE MESH TRAPS
iO
.I-'" .....,
>_.~
5 I 0
0
2
3
4
,
I
0
2
2
4
NUMBER OF TUNNELS PER TRAP
Fig. 3. Mean number of P. platyceros caught by three trap types, with varying number of tunnels, at two soak times. 30"
ac
~ 0----.0
301 "--
SHORT TUNNEL TRAPS INSERTTUNNEL TRAPS
2.
Z~----'%
20"
20
I0-
I0
MEDIUM T U N N E L TRAPS LONG T U N N E L TRAPS
" 5a_
+1
5~
m <•
tI
0 24
48
72
96
24
48
72
96
SOAK TIME (h}
Fig. 4. Mean number of P. platyceros caught by similar sized conical nesting traps with different tunnel lengths at four soak times.
nism between prawns crowded in a trap or between captured prawns and those trying to enter the trap could determine saturation level. With regard to crabs, Miller (1979) and Breen (1985) proposed that saturation was caused by captured individuals intimidating those trying to enter the trap. Increasing prawn trap volume decreases the chance of encounters and increases the saturation level. Volume is an important variable when standardizing a unit of effective fishing effort.
76 TABLE IV Kruskal-Wallis results of the ranks of the number of Pandalusplatyceros caught as the dependent variable against soak time and tunnel number according to trap type: (A) conical nesting; (B) round 'Pardiac'; (C) square wire mesh Source
(A)
(B)
(C)
Kruskal-Wallis test df
Statistic
P
Conical nesting traps Soak time Tunnel number Soak/tunnel interaction
1 2 5
2.16 3.13 8.38
0.1417 0.2095 0.1365
Round 'Pardiac' Soak time Tunnel number Soak/tunnel interaction
1 2 5
2.00 1.03 3.38
0.1574 0.5964 0.6414
Square wire mesh Soak time Tunnel number Soak/tunnel interaction
1 1 3
0.09 0.08 0.41
0.7618 0.7778 0.9373
T h e n u m b e r of t u n n e l s w i t h i n t h e t h r e e t r a p s d e s i g n s u s e d did n o t a f f e c t p r a w n c a t c h e s a t e i t h e r soak. S i m i l a r s a t u r a t i o n levels for a t r a p t y p e w e r e r e a c h e d i r r e s p e c t i v e of t h e n u m b e r of t u n n e l s . W e c o n c l u d e t h a t t h e n u m b e r of t u n n e l s is n o t a n i m p o r t a n t v a r i a b l e for c a l c u l a t i n g t h e e f f i c i e n c y of p r a w n traps. TABLE
V
Characteristics of traps used in the tunnel length experiment Tunnel type
Tunnel characteristics Length
Number
Internal opening
External opening
(cm) Shape
Area
Shape
(cm ~) S M L Inserta
10 18 24 20
3 3 3 3
Rectangular Rectangular Rectangular Round
495 495 495 64
Area
(cm 2) Round Round Round Round
57 57 57 57
"Little-tapering,cylindricaltunnels inserted through a hole cut in the trap's side mesh. All traps were the conical nesting type (volume = 110 l;see Table I), composed of a stainlesssteel frame covered with black, knotted nylon 'herringbunt' web of 2.4 c m stretch mesh, knot-to-knot.
77 TABLE VI ANOVAresults using fourth root of the numberof Pandulus platyceros caughtas the dependent variable against soaktime and tunnel length in similar sizedconicalnestingtraps Source
df
Mean square
F
Soak time Tunnel length Soak/tunnel interaction Error
3 3 9 368
0.29445 0.79458 0.38647 0.15712
1.87 5.06** 2.46**
**P<0.01. Tunnel lengths of 10-24 cm in conical nesting traps did not significantly influence prawn catches. Ronholt (1974) obtained similar results with 23-33cm tunnels for a different trap design. Insert tunnels (20 cm long), however, which had relatively small (Table V) external openings flush with the trap exterior, were in traps which caught significantly fewer prawns than all the other traps with larger external openings. Kessler (1969) found that prawns approaching traps along the substrate tend to be diverted by vertical trap walls, but are more readily led into a trap by gradually tapering tunnel walls from wide external openings. After long soaks, the insert tunnel traps eventually reach a saturation level similar to the other traps. Wide external tunnel openings (tapering to smaller internal openings ) are a more important criteria than tunnel length in determining catch. Morgan (1979) used categories to develop a standardized unit of effective fishing effort, of which two were the fishing power of the gear and the fishing time, i.e. soak time during which the gear is actively fishing. Fishing power variability is caused by two distinguishable sets of factors: the differences between gear type saturation levels and the efficiency with which a gear type reaches its saturation level. In Simpson's (1975) discussion on effort measurement in crustacean trap fisheries, changing trap type altered catch per trap by a factor of two in an area with unchanged stock abundance. We show here variations in saturation levels caused by trap volume. In addition, the fishing power of a single gear type can be related to the design of tunnel entrance, as shown in the length of tunnel experiment. Fishing ( soak ) time during which gear is actively fishing is a variable in all experiments. Soak time had a significant effect on catches, either alone or after interaction with variables such as trap volume and tunnel length. Munro (1974) indicated that the interactions of soak time, gear design, ingress and escapement combine to produce unique asymptotic catch curves in trap fisheries. Munro (1974) and Bennett and Brown (1979) caution that evaluating the influence of soak time on trap catches for stock estimates requires care. For example, Munro (1974) stressed that catch per day soaked can be misleading
78
when dealing with asymptotic catch curves. He suggested an index of 'availability', equal to the mean daily rate of ingress of animals into traps, for a comparable measure of stock density. It is likely that the asymptotic catch was reached before 24 h in some of our experiments (Figs. 2, 3 and 4). Boutillier (1986) found, in some instances, P. platyceros catch per hour soaked declined significantly after 3-6 h. The industry soak standard is ~ 24 h, which may confound interpretation of their catch records since the catch may have reached an asymptotic level. With these experiments and those of Boutillier (1985, 1986) and Boutillier and Sloan (in press ), the groundwork has been laid to develop a data collection system, from either fishermen's log books or test fishing, which can be interpreted to reflect changes in prawn population density rather than just changes in the fishing process. ACKNOWLEDGEMENTS
Our thanks to the skipper and mate of the "M.V. CALIGUS", G. Brown, W. Carolsfeld, W. Harling, S. Head, and D. Heritage for field assistance. G.S. Jamieson, B.D. Smith and S.L. Swarbrick kindly commented on various drafts.
REFERENCES Bennett, D.B. and Brown, C.G., 1979. The problems of pot immersion time in recording and analyzing catch-effort data from a trap fishery. Rapp. P.-V. Reun. Cons. Int. Explor. Mer, 175: 186-189. Boutillier, J.A., 1986. Fishing effort standardization in the British Columbia prawn (Pandalus platyceros) trap fishery. Can. Spec. Publ. Fish. Aquat. Sci., 92: 176-181. Boutillier, J.A., 1985. Important variables in the definition of effective fishing effort for the British Columbia prawn (Pandalus platyceros) trap fishery. J. Shellfish Res., 5: 13-19. Boutillier, J.A. and Sloan, N.A., in press. Trap mesh selectivity in relation to the legal size regulation for prawn (Pandalus platyceros) in British Columbia. J. Shellfish Res., Breen, P.A., 1985. Crab gear selectivity studies in Departure Bay. Can. MS Rep. Fish. Aquat. Sci., 1848: 21-39. Dahlstrom, W., 1963. Prawn shrimp. Calif. Dep. Fish. Game Cruise Dep. 63-A-1. Dixon, W.J. (Editor), 1985. BMDP Statistical Software. (1985 printing). University of California Press, Berkeley, 733 pp. Kessler, D.W., 1969. Test-tank studies of shrimp -pot efficiency. Fish. Ind. Res. (U.S. Fish. Wildl. Serv.), 5: 151-160. Miller, R.J., 1979. Saturation of crab traps: reduced entry and escapement. J. Cons. Int. Explor. Mer, 38: 338-345. Morgan, G.R., 1979. Trap response and the measurement of effort in the fishery for the western rock lobster. Rapp. P.-V. Reun. Cons. Int. Explor. Mer, 175: 197-203. Munro, J.L., 1974. The mode of operation of Antillean fish traps and the relationships between ingress, excapement, catch, and soak. J. Cons. Int. Explor. Met, 35: 337-350. Ricker, W.E., 1975. Computation and interpretation of biological statistics of fish populations. Bull. fish. Res. Board Can., 191:382 pp.
79 Ronholt, L.L., 1974. A study of the relative efficiency of shrimp pots for harvesting the spot shrimp, Pandalus platyceros, in southeastern Alaskan waters. M.Sc. Thesis, University of Washington, Seattle, 101 pp. Simpson, A.C., 1975. Effort measurement in the trap fisheries for crustacea. Rapp. R.-V. Reun. Cons. Int. Explor. Mer, 168: 50-53. Zar, J.H., 1984. Biostatistical Analysis. (Second edition). Prentice-Hall Inc., New Jersey, 718 pp.