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The response of spanner crabs (Ranina ranina) to tangle nets – behaviour of the crabs on the nets, probability of capture and estimated distance of attraction to bait
Fisheries Research 41 (1999) 37±46
The response of spanner crabs (Ranina ranina) to tangle nets ± behaviour of the crabs on the nets, probability of capture and estimated distance of attraction to bait B.J. Hill, T.J. Wassenberg* CSIRO Division of Marine Research, PO Box 120, Cleveland 4163, Australia Received 5 May 1998; accepted 30 November 1998
Abstract We made TV observations of spanner crabs (Ranina ranina) attracted to a baited tangle net designed to ®sh the crabs commercially off the East Coast of Australia. The study area has moderate bottom water currents, mean speed 9.6 cm sÿ1, and all spanner crabs arriving at the nets did so from down current of the nets. The rate at which spanner crabs responded to the bait rose to a peak between 12 and 21 min after the net was placed on the bottom and then declined. From ®eld data on current speed together with laboratory data on speed of movement of spanner crabs we estimated that in 30 min, crabs could be attracted from as far as 70 m from the bait. 80% of crabs that reached the net crossed on to it and 37% of these reached the bait. Spanner crabs spent on an average only 114 s at the bait after which they attempted to leave. Some became entangled while crossing the net or while feeding but after the net had been on the seabed for 30 min, only 7% of spanner crabs that had crossed onto the net were still on it. This suggests low retention by this gear. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Tangle nets; Spanner crabs; Distance of attraction to bait; Catchability; Behaviour
1. Introduction Spanner crab (Ranina ranina) ®sheries have expanded rapidly as a consequence of rising prices and consumer demand for live spanner crabs, a product made possible by advances in methods for live transport as well as the availability of rapid air freight. This has put pressure on spanner crab stocks in many parts of the Indo West-Paci®c region. The catch off Queensland, east coast of Australia, rose from 400 t in 1983 to 1740 t in 1993 (Queensland Department of *Corresponding author. Tel.: +61-73826-7217; fax: +61-738267222; e-mail: [email protected]
Primary Industries logbook data) and new management arrangements are being developed to deal with the situation. Many ®sheries show patterns of growth in effort and landings followed by declining catches even if effort is maintained or increased. It is dif®cult to manage ®sheries that reach this stage because of economic and social implications of management decisions (John, 1994). Consequently ®sheries that are growing need close monitoring so that adequate controls can be introduced before capacity has exceeded sustainable levels. Spanner crabs off the East Coast of Australia live on sandy substrates at depths of 20±80 m (Kennelly, 1992) and are ®shed by means of tangle nets made
0165-7836/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0165-7836(99)00009-0
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B.J. Hill, T.J. Wassenberg / Fisheries Research 41 (1999) 37±46
of mono®lament mesh fastened across a metal frame with bait attached to the centre. They are readily attracted to a bait and they are particularly susceptible to entanglement in nets because their dactyls are ¯attened and have narrow joints (Bourne, 1922). The nets are ¯at and easy to transport and so large numbers can be worked from a small boat. Kennelly (1989) reported that tangle nets catch spanner crabs at a rate of around 10 crabs per hour. Little is known about the biology of spanner crabs in the wild, especially their behaviour with respect to the gear used to capture them. Most knowledge has been inferred from catch data (e.g. Kennelly and Watkins, 1994). Traps and pots rely on the target species responding positively to a bait scent and so only those animals that receive the scent and want to feed will be attracted. Feeding in crustaceans is affected by a range of external factors such as temperature and light as well as physiological conditions including the stage in the moult cycle and the sex of the animal (Krouse, 1989). In the case of spanner crabs, adult males do not feed for about 52 days around their annual moult, whereas females stop feeding for only about 22 days (Skinner and Hill, 1987). This will bias the sex composition of spanner crab catches at certain times of the year. We do not know whether the response behaviour of the spanner crabs to the bait and nets affects the size and sex composition of the catches. We used underwater TV to observe the behaviour of spanner crabs on and around baited tangle nets. Direct observations also enabled us to compare the number of spanner crabs caught with the number attracted to the net and thus to calculate the retention ef®ciency of the net. We used the technique described by Sainte-Marie and Hargrave (1987) for calculating the distance over which spanner crabs are attracted to bait. This was based on data on the time of arrival of spanner crabs at the bait, speed and direction of water currents at the time of trapping with laboratory-derived data on the response time of the spanner crabs to a food stimulus and the speed at which spanner crabs move over the bottom. 2. Materials and methods 2.1. Field study The study was carried out on the east coast of Queensland to the north and east of Moreton Island
(278100 S, 1538250 E), 1986±1992. Commercial ®shers use a variety of shapes and sizes of tangle nets covered by a range of mesh sizes to catch spanner crabs. We used one of the designs in common use in southern Queensland. This consisted of a circular 1.2 m diameter net having a rim of 12.5 mm diameter steel rod and covered with a single layer of 53 mm stretched mesh net made of 2 mm diameter mono ®lament nylon. One of these nets was fastened across the base of a four-legged frame that supported a television camera 2 m above the bottom. A piece of ®sh (usually Mugil cephalus) was attached to the centre of the net as bait. This is a daytime ®shery so we made all observations in daylight. The camera had a high sensitivity to light and could be used with ambient daylight at all depths down to the maximum worked; this avoided the need for arti®cial light that might disturb the spanner crabs. The wide-angle lens of the camera gave a ®eld of view of approximately 23 m at a distance of 2 m off the seabed. This was suf®cient to show the whole net together with an area outside of the net so that spanner crabs approaching the net could be seen before they encountered the rim. A piece of tufted cord attached near the centre of the net indicated the direction of the water current over the net. A cable carried power to the camera and the television signal to a monitor and recorder in the boat. The frame, with net and camera, was left in place on the bottom for 30 min and the TV signal was recorded at the surface. A Savonius rotor current meter that averaged water speed at 3 min intervals was put onto the bottom about 6 m from the camera each time the system was deployed. 2.2. Analysis of TV information The TV tapes were analysed on a tape recorder allowing a range of playback speeds from single frame operation up to six times normal. The recorder had a digital timer (reading to 1 s) which was used to measure the duration of aspects of the behaviour of spanner crabs. For each spanner crab we recorded the length of time between the start of the experiment (when the net reached the seabed) and the time of arrival of the crab at the rim of the net as well as the direction (in 308 sectors) of arrival. When large numbers arrived at the rim of the net, crabs that were already there sometimes obstructed the new arrivals.
B.J. Hill, T.J. Wassenberg / Fisheries Research 41 (1999) 37±46
These later arrivals occasionally responded by backing away and circling around the net. Some of them disappeared from the ®eld of view and would be counted as new arrivals if they subsequently returned to the net. We compensated for this source of bias by subtracting from the number of arrivals, all spanner crabs that left the ®eld of view without crossing over the rim. This problem arose in sets in which a large number of spanner crabs responded. In one case in which there were 226 arrivals, 34 crabs left the ®eld of view before crossing the rim and we interpreted this set as having attracted 192 individuals. In a set with 125 arrivals, six crabs left the ®eld of view whereas in a set with 35 arrivals, only two left the ®eld of view. The number of crabs that arrived at the net would be biased upwards if crabs that had crossed onto the net and then left it subsequently returned to the net. If a crab crossed over the rim onto the net, we recorded the time of crossing. If it reached the bait we also recorded the time taken to cross the net to the bait and the time spent at the bait. If the crab escaped, we noted the time taken to cross the net from the bait to the rim and the direction of leaving. The direction of the current varied during any particular set and so we noted the direction every three minutes and took the most frequent direction as being the main direction for each set. The direction of arrival and departure of spanner crabs was recorded relative to this main current direction. To avoid problems associated with analysis of angular data (Cain, 1989), we used the V test, as described by Schmidt-Koenig (1975), to test whether the directions of arrival and leaving were uniformly distributed or clustered around the direction of the current. The exact angle of arrival or departure of the crabs was not recorded but only the number in each of 12 sectors. To test whether this method of recording was adequate, the analysis was done twice: 1. assuming that all spanner crabs arrived or departed at the middle of the sector; 2. assuming that the arrivals and departures were uniformly distributed within each sector. The difference in results was negligible indicating that counts in sectors were an adequate way of measuring angle of arrival or departure. We carried out a separate experiment to check whether the size and sex ratio of spanner crabs caught
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by tangle nets was the same as that of those attracted to bait. The net was removed and replaced by a line attached between diagonally opposite legs of the camera frame. A piece of bait and a 100 mm long plastic rod were tied to the centre of this line. The position of the camera on the frame was lowered to 1 m above the seabed to give a closer view of spanner crabs attracted to the bait. We used sliding callipers to measure the apparent carapace width of crabs on the television monitor screen. This measurement was corrected using a scaling factor obtained by measuring the length of the image of the 100 mm rod. The image on a television monitor is distorted towards the edge and so only crabs at the centre of the monitor were measured. Carapace length rather than width is usually measured in studies of spanner crabs, but when the animals are alive, they often stand with their body angled upward. This would cause a large error in a carapace length measurement viewed from above and so we used carapace width. Individuals with a carapace width larger than about 50 mm can be sexed from the size and shape of the horns on the carapace and we used this characteristic for sexing animals on the video. They were excluded from the analysis if this was not possible because of the angle of the crab or because the carapace was damaged or because the crabs were too small. Each time the camera was lowered in this con®guration, three baited tangle nets were laid at least 100 m from the boat and retrieved after 40±60 min. All spanner crabs caught on these nets were measured and sexed. We used a square root transformation to normalise the data on size composition because it was skewed. We then used an analysis of variance to compare the size and sex of spanner crabs attracted to bait under the camera with those caught on nets at the same time. We used a Chi-Square test to compare the observed number of males and females caught on tangle nets with the expected numbers derived from the experiment in which no net was used. 2.3. Laboratory study The speed at which spanner crabs moved over the bottom was measured in a 12 m long, 1 m wide tank containing 750 mm depth of sea water (temperature 21±238C) and a 150 mm deep layer of sand. Twelve crabs (7 females and 5 males) in the size range 60±
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B.J. Hill, T.J. Wassenberg / Fisheries Research 41 (1999) 37±46
111 mm carapace width, were collected from tangle nets and taken to the laboratory in drums of seawater. The carapace width of each crab was measured; it was labelled by gluing a numbered plastic disc to the carapace and then put into the tank. The crabs were allowed to settle down for two days con®ned to a 1.5 m long section of the tank by a barrier. Following this settling in period, on each of 10 mornings the barrier was lifted and a piece of ®sh was dragged along the length of the tank. Between 1 and 6 crabs (mean 4.2) emerged each day and moved 9 m down the tank after the food stimulus was introduced. The identity number of each crab that responded and the time to travel 9 m along the tank was noted. The crabs were then returned to the space behind the barrier. We used an F-statistic to test whether there was a relationship between speed and carapace width. 2.4. Estimation of distance of attraction We estimated the maximum distance (dmax) from which most spanner crabs could have been attracted to the bait in each set using the relationship dmax t Vcurrent Vcrab = Vcurrent Vcrab ; where dmax is the attraction distance in metres, t the time period in min (30 min in these experiments), Vcurrent the mean speed of current in m minÿ1 and Vcrab is the mean speed of crabs in m minÿ1 (derived experimentally above). We used average spanner crab speeds (as absolute values) in this estimate and so the fastest moving individuals could have come from a greater distance. Fluctuations in current speed could also have spread the bait scent further than is indicated by using average current speeds. 3. Results 3.1. Field study Although the underwater TV system generally worked well, on some occasions, the drag of the current on the power and signal cable caused the camera to fall over. A system using a self-contained camera would remove the need for the cable and probably remedy this problem. We obtained 12 complete and one incomplete 30 min sets, but on two of
these no spanner crabs appeared and so the results from 11 sets in which crabs responded are used in some analysis but only 10 for the numbers arriving and leaving. The harmonic mean current speed was 9.6 cm sÿ1 (5.76 m minÿ1) and ranged between 3.3 and 15.6 cm sÿ1. The current was rarely constant for the 30 min period of a set, the coef®cient of variation ranged from 8% to 62%, but it was mostly between 20% and 40%. Although the current always ran in one main direction, it ¯uctuated around this direction as can be seen in Fig. 1(a) and for 99% of the time it was in a 908 arc. There was a signi®cant negative relationship between the direction from which the crabs arrived and current direction (u22.3, p<0.01) showing that they were moving into the current (Fig. 1(b)). Few crabs arrived in the ®rst 3 min after the net reached the bottom (Table 1). The mean time of ®rst arrival was 7 min 5 s (Table 2) with the shortest time 2 min 21 s. The numbers arriving then rose rapidly to reach a peak between 15 and 21 min after which the rate of arrival declined (Table 2). On an average, 36.7 spanner crabs appeared at the net but the number ranged from 2 to 192 and this wide range resulted in a large standard deviation. As mentioned above, on two occasions no crabs responded. If these two sets are included, then the average number arriving was 31.0. On the set in which 192 crabs appeared, it was not possible to distinguish some aspects of their behaviour because they climbed on top of each other. In this case we recorded the time and direction of arrival of each crab, the number that crossed onto the net as well as
Table 1 The mean numbers (and standard deviation) of spanner crabs (R. ranina) arriving at a baited tangle net in successive three minute periods based on 10 complete 30 min sets Time (min)
Mean
Standard deviation
3 6 9 12 15 18 21 24 27 30
0.3 0.8 2.3 4.1 7.4 6.8 7.3 5.9 5.3 3.7
0.5 1.6 2.8 6.9 13.5 11.8 11.6 9.5 11.2 7.2
B.J. Hill, T.J. Wassenberg / Fisheries Research 41 (1999) 37±46
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Fig. 1. (a) The percentage frequency of direction in 308 sectors of water currents over the tangle net during field experiments (92 readings). (b) The direction from which spanner crabs arrived at the rim of the net (n404) relative to the direction of the water current over the net. (c) The direction in which spanner crabs left the net (n283 measurements).
the direction and time of departure of those that clearly crossed onto the net. Some crossed the rim of the net immediately, but most stopped at the rim for a short time before cross-
ing (Table 2). The mean time taken to cross the net from the rim to the bait was 22.6 s, but the distribution of times was highly skewed and 51% took 5 s or less. Once they were on the net, progress was often slowed
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B.J. Hill, T.J. Wassenberg / Fisheries Research 41 (1999) 37±46
Table 2 Results of underwater video observations of the behaviour of spanner crabs (R. ranina) on tangle nets Behaviour
Outcome or sample data
Number of sets Mean time of arrival of first crab at net Total number seen arriving at net Mean number seen arriving at net Percentage crossing onto net Mean time before crossinga Percentage reaching baita Mean time taken to reach baita Mean time at baita Mean time to cross the net after feedinga Percentage of crabs that crossed onto the net and were still there after 30 min
11 sets 368 s (SD165 s) (n10 sets) 404 (n11 sets) 36.7 (SD60.6) (n11 sets) 81% (n404 crabs) 13.0 s (SD24.7) (n172 crabs) 37% (n218 crabs) 22.6 s (SD32.5) (n75 crabs) 113.7 s (SD100.4) (n67 crabs) 48.1 s (SD92.9) (n62 crabs) 7% (n329 crabs)
The value of n varies for different reasons: in one set the first minute of recording was lost and so the time of the first arrival was not obtained; crabs sometimes climbed over each other and so we could not accurately determine when some crossed onto the net; not all spanner crabs that crossed the net managed to reach the bait; not all spanner crabs that reached the bait fed; and not all spanner crabs that left the bait managed to escape from the net. a The set in which 192 crabs arrived at the net is not included in calculations.
because some crabs became entangled or other spanner crabs obstructed them and some never reached the bait. If they did, they fed rapidly, and on average, spent less than 2 min at the bait. Many crabs became temporarily entangled in the mesh while feeding and this slowed their departure. On average it took about twice as long to cross the net from the bait to the rim as it did to cross from the rim to the bait. Crabs that were not entangled walked or swam off the net. They left the net in all directions although fewer did so into the current (Fig. 1(c)). This bias was suf®cient to give a signi®cant relationship (u2.95, p<0.01) between the direction in which the crabs left the net and current direction ± the mean result was that the crabs departed the net in the same direction as the current ¯owed. The low value of u re¯ects the widely dispersed directions taken by departing crabs. Although crabs arrived at the net throughout the period of the set, others were leaving and so the number on the net rose only slowly. Towards the end of the 30 min set, the rate of departure exceeded the rate of arrival and the number on the net dropped (Fig. 2). Only 23 crabs out of the total of 329 that crossed onto the net in the 11 sets were still on it when it was lifted. If it is assumed that all of these would have been caught when the net was lifted, the capture rate would be 7% of those that crossed onto the net and an even lower proportion of those attracted to the bait.
The size of spanner crabs attracted to the bait without a net and the sizes of those caught on adjacent nets set at the same time, are shown in Fig. 3. There is no signi®cant difference between the size of females caught on tangle nets (mean 68.7 mm, SD 7.3, n71) and those attracted to a bait not attached to a net (mean 71.8 mm, SD 8.5, n202) (F2.40). However the F-test indicated that the size of males caught on tangle nets (mean 77.2 mm, SD 12.8, n55) was signi®cantly smaller than those attracted to a bait not attached to a net (mean 81.3 mm, SD 12.1, n142) (F5.08). The difference is minor and given measurement errors and variations in populations, may not be biologically signi®cant. Tangle nets did not show a sex bias: 59% of 344 spanner crabs attracted to a bait were females, compared to 56% of the 126 crabs caught on tangle nets set at the same time. 3.2. Laboratory study Two of the 12 crabs in the tank did not respond to the bait. The remaining 10 walked the full 9 m in the tank at least once during the 10 experiments. Some crabs stopped part way along the tank and either buried, or moved back towards the start. The mean speeds at which the individual spanner crabs travelled are given in Table 3. Speeds varied quite markedly and there was no relationship between speed and carapace width (F0.007, p0.94, n38). We assumed for the
B.J. Hill, T.J. Wassenberg / Fisheries Research 41 (1999) 37±46
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Fig. 2. Cumulative numbers of spanner crabs arriving, leaving and remaining on a baited tangle net. Average figures based on 11 sets.
Table 3 Mean speed of travel of 10 spanner crabs (R. ranina) over 9 m in an experimental tank
Table 4 Estimated maximum distances from which spanner crabs (R. ranina) could have been attracted to a baited net
Carapace width (mm)
Set number
Current speed (m minÿ1)
Distance of attraction (m)
1 3 4 5 6 7 9 10
4.3 4.3 2 5.6 6.3 7.1 8.5 7.9
61 61 40 68 72 75 79 78
n
Speed ÿ1
62 65 70 71 74 81 86 95 105 111 82.0
ÿ1
cm s
m min
4 1 3 3 5 2 7 4 3 5
5.8 4.7 10.1 9.0 7.0 6.8 3.2 7.3 2.2 9.3
3.5 2.8 6.1 5.4 4.2 4.1 1.9 4.4 1.3 5.6
37
Mean 6.4
n is the number of times each individual responded out of 10 trials.
purposes of estimating the distance over which crabs are attracted to bait, that they move at an average speed of 6.4 cm sÿ1, which is equivalent to 3.84 m minÿ1.
Note that the current meter failed on sets is not shown in this list.
3.3. Estimation of distance of attraction The current meter failed on three of the sets giving eight sets with a complete set of current speed data. Using the formula given above and the ®eld and laboratory data, we calculated that in 30 min, spanner crabs could be attracted from a maximum distance of around 40±80 m downstream of the bait (Table 4).
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B.J. Hill, T.J. Wassenberg / Fisheries Research 41 (1999) 37±46
Fig. 3. Size composition of (a) female and (b) male spanner crabs attracted to bait with no net and those caught on a baited tangle net. n202 females at bait without a net, n71 captured on a baited net. n142 males at bait without a net, n55 captured on a baited net.
4. Discussion In interpreting our results, we have assumed a scenario in which on each occasion a net is set, a number of R. ranina in the area will be exposed to the bait scent. Some of these respond to the scent and move up current to reach the net within the soak time. These crabs are available to be caught but will be captured only if they cross onto the net and become entangled. The process starts when the net reaches the bottom. The mean time taken for the ®rst spanner crab to arrive at the net was 7 min 5 s after the net arrived
on the seabed. This delay may be due to crabs that were very close to the bait being inhibited from responding rapidly because of disturbances such as the boat overhead and the arrival of the camera frame and net on the bottom. Some of the delay is probably a function of the time taken by the crabs to respond to a bait scent. Skinner and Hill (1987) found that on an average 37% of spanner crabs responded within 4.5 min of being exposed to a bait scent. The number of spanner crabs arriving at the net rose to a peak between 12 and 21 min after the start. After this, it fell presumably because crabs coming to the bait are
B.J. Hill, T.J. Wassenberg / Fisheries Research 41 (1999) 37±46
depleting the subset of the population in the area exposed to the bait scent and so there are progressively fewer available to respond. Leaching of the bait is probably not a factor in this ®shery because of the short soak time (see Zimmer-Faust and Case, 1983 for a discussion on bait leaching). Assuming the average speed of spanner crabs when moving towards a food sources is 3.8 m minÿ1, they could be attracted from up to 69 m away in a 30 min set. However, our laboratory experiments showed that some crabs can move at speeds up to 6.1 m minÿ1 and so some spanner crabs might be attracted from further than suggested by the average speed. Sinoda and Kobayasi (1969) assumed that the area ®shed by a baited trap for spider crabs was circular. This is unlikely to apply to situations in which there is a current. Skinner and Hill (1987) found that R. ranina responded to the presence of food by emerging from the sand and moving up current. In the present study, spanner crabs arriving at the net were moving into the current. The effective distance the bait scent travels is obviously related to its concentration and the current velocity. Our derived data suggests a direct correlation between current velocity and distance of attraction but this needs further investigation. We found that for about 60% of the time in our study area, the current direction was from one 308 arc, and for nearly all of the rest of the time it came from adjacent 308 arcs. Under these conditions, most of the bait odour would spread out in a triangular plume subtending a 908 angle from the bait and this is probably the approximate shape of the area of attraction of the bait. Experimentally, not all crabs responded to bait. This also likely to apply in the ®eld. Thus not all spanner crabs around a net are immediately vulnerable to capture, only those down current of it. Obviously this could change suddenly with a change in direction of water ¯ow. The level of escape from the design of tangle net that we used appears to be very high. On an average only 7% of crabs that crossed onto the net were still there after 30 min. In the case when 192 individuals were recorded as arriving at the net, 139 crossed over the rim, but 20 was the highest number of crabs on the net at any one time and only 15 were still on the net after 30 min ± 7.8% of the arrivals. The low retention rate may have been due to the way the net was hung, we noticed that the mesh tended to become smothered
45
in sand ± often because of crabs attempting to bury while on the net ± and this reduced the probability of entangling other crabs. Nevertheless, on an average there were only nine crabs on the net after 30 min. This is close to the catch rate of 10 spanner crabs per hour reported by Kennelly (1989) and so our tangle net was catching at a similar rate. The proportion that is captured could be increased if more spanner crabs could be prevented from escaping. This can be achieved by having a double layer of mesh (Kennelly and Craig, 1989). However that would make it more dif®cult to disentangle the crabs and increases the likelihood that crabs discarded because they are undersized or ovigerous, will lose limbs in the process. Kennelly et al. (1990) found signi®cant mortalities in spanner crabs that had lost limbs. This has been con®rmed by Kirkwood and Brown (1998) who reported varying mortalities depending on the extent of damage. Removal of a single dactylus resulted in 20% mortality while removal of a single cheliped led to 90% mortality. Consequently, doubling of the mesh is not favoured as a strategy. We conclude that the equipment used in this ®shery is not ef®cient. Given the increasing ®shing pressure being exerted on stocks, development of a more ef®cient method of capturing the spanner crabs may be inappropriate. However, a catching method that eliminates limb damage is needed urgently. Acknowledgements The Queensland Department of Primary Industries (QDPI) assisted in this study by making a research vessel available for some of the ®eldwork. We also wish to thank Dr. Ian Brown of QDPI for fruitful discussions about the project, assistance in ®eldwork and valuable comments on the paper. Dr. YouGan Wang of the CSIRO Biometrics Unit kindly provided assistance with the analysis of the angular data and Dr. David Die made useful comments about the paper. References Bourne, G.C., 1922. A study in carcinology. J. Linn. Soc. (Zool) 35, 25±78. Cain, M.L., 1989. The analysis of angular data in ecological field studies. Ecol. 70, 1540±1543.
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John, J. (Ed.), 1994. Managing redundancy in over exploited fisheries. World Bank Discussion papers; 240 III Series: Fisheries Series 128. Kennelly, S.J., 1989. Effects of soak-time and spatial heterogeneity on sampling populations of spanner crabs Ranina ranina. Mar. Ecol. Prog. Ser. 55, 141±147. Kennelly, S.J., 1992. Distributions, abundances and current status of exploited populations of spanner crabs Ranina ranina off the east coast of Australia. Mar. Ecol. Prog. Ser. 85, 227±235. Kennelly, S.J., Craig, J.R., 1989. Effects of trap design, independence of traps and bait on sampling populations of spanner crabs Ranina ranina. Mar. Ecol. Prog. Ser. 51, 49±56. Kennelly, S.J., Watkins, D., 1994. Fecundity and reproductive period, and their relationship to catch rates of spanner crabs, Ranina ranina, off the east coast of Australia. J. Crust. Biol. 14, 146±150. Kennelly, S.J., Watkins, D., Craig, J.R., 1990. Mortality of discarded spanner crabs Ranina ranina (Linnaeus) in a tangle net fishery ± laboratory and field experiments. J. Exp. Mar. Biol. Ecol. 140, 39±48. Kirkwood, J.M., Brown, I.W., 1998. Effect of limb damage on the survival and burial time of discarded spanner crabs Ranina ranina (Linnaeus). Mar. Freshwater Res. 49, 41±45.
Krouse J.S., 1989. Performance and selectivity of trap fisheries for crustaceans. In: Caddy, J.F. (Ed.), Marine Invertebrate Fisheries: Their Assessment and Management. Wiley, New York, pp. 327±374. Sainte-Marie, B., Hargrave, B.T., 1987. Estimation of scavenger abundance and distance of attraction to bait. Mar. Biol. 94, 431±443. Schmidt-Koenig, K., 1975. Migration and homing in animals. Zoophys and Ecol, vol. 6. Springer, Berlin, pp. 1±99. Sinoda, M., Kobayasi, T., 1969. Studies of the fishery of Zuwai crab in the Japan Sea VI. Efficiency of the toyama kago (a kind of crab trap) in capturing beni-zuwai crab. Bull. Jpn. Soc. Sci. Fish 35, 948±956. Skinner, D.G., Hill, B.J., 1987. Feeding and reproductive behaviour and their effect on catchability of the spanner crab Ranina ranina. Mar. Biol. 94, 211±218. Zimmer-Faust, R.K., Case, J.F., 1983. A proposed dual role of odour in the foraging by the California spiny lobster, Panulirus interruptus (Randall). Biol. Bull. Mar. Biol. Lab Woods Hole 164, 341±353.
The response of spanner crabs (Ranina ranina) to tangle nets ± behaviour of the crabs on the nets, probability of capture and estimated distance of attraction to bait B.J. Hill, T.J. Wassenberg* CSIRO Division of Marine Research, PO Box 120, Cleveland 4163, Australia Received 5 May 1998; accepted 30 November 1998
Abstract We made TV observations of spanner crabs (Ranina ranina) attracted to a baited tangle net designed to ®sh the crabs commercially off the East Coast of Australia. The study area has moderate bottom water currents, mean speed 9.6 cm sÿ1, and all spanner crabs arriving at the nets did so from down current of the nets. The rate at which spanner crabs responded to the bait rose to a peak between 12 and 21 min after the net was placed on the bottom and then declined. From ®eld data on current speed together with laboratory data on speed of movement of spanner crabs we estimated that in 30 min, crabs could be attracted from as far as 70 m from the bait. 80% of crabs that reached the net crossed on to it and 37% of these reached the bait. Spanner crabs spent on an average only 114 s at the bait after which they attempted to leave. Some became entangled while crossing the net or while feeding but after the net had been on the seabed for 30 min, only 7% of spanner crabs that had crossed onto the net were still on it. This suggests low retention by this gear. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Tangle nets; Spanner crabs; Distance of attraction to bait; Catchability; Behaviour
1. Introduction Spanner crab (Ranina ranina) ®sheries have expanded rapidly as a consequence of rising prices and consumer demand for live spanner crabs, a product made possible by advances in methods for live transport as well as the availability of rapid air freight. This has put pressure on spanner crab stocks in many parts of the Indo West-Paci®c region. The catch off Queensland, east coast of Australia, rose from 400 t in 1983 to 1740 t in 1993 (Queensland Department of *Corresponding author. Tel.: +61-73826-7217; fax: +61-738267222; e-mail: [email protected]
Primary Industries logbook data) and new management arrangements are being developed to deal with the situation. Many ®sheries show patterns of growth in effort and landings followed by declining catches even if effort is maintained or increased. It is dif®cult to manage ®sheries that reach this stage because of economic and social implications of management decisions (John, 1994). Consequently ®sheries that are growing need close monitoring so that adequate controls can be introduced before capacity has exceeded sustainable levels. Spanner crabs off the East Coast of Australia live on sandy substrates at depths of 20±80 m (Kennelly, 1992) and are ®shed by means of tangle nets made
0165-7836/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0165-7836(99)00009-0
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B.J. Hill, T.J. Wassenberg / Fisheries Research 41 (1999) 37±46
of mono®lament mesh fastened across a metal frame with bait attached to the centre. They are readily attracted to a bait and they are particularly susceptible to entanglement in nets because their dactyls are ¯attened and have narrow joints (Bourne, 1922). The nets are ¯at and easy to transport and so large numbers can be worked from a small boat. Kennelly (1989) reported that tangle nets catch spanner crabs at a rate of around 10 crabs per hour. Little is known about the biology of spanner crabs in the wild, especially their behaviour with respect to the gear used to capture them. Most knowledge has been inferred from catch data (e.g. Kennelly and Watkins, 1994). Traps and pots rely on the target species responding positively to a bait scent and so only those animals that receive the scent and want to feed will be attracted. Feeding in crustaceans is affected by a range of external factors such as temperature and light as well as physiological conditions including the stage in the moult cycle and the sex of the animal (Krouse, 1989). In the case of spanner crabs, adult males do not feed for about 52 days around their annual moult, whereas females stop feeding for only about 22 days (Skinner and Hill, 1987). This will bias the sex composition of spanner crab catches at certain times of the year. We do not know whether the response behaviour of the spanner crabs to the bait and nets affects the size and sex composition of the catches. We used underwater TV to observe the behaviour of spanner crabs on and around baited tangle nets. Direct observations also enabled us to compare the number of spanner crabs caught with the number attracted to the net and thus to calculate the retention ef®ciency of the net. We used the technique described by Sainte-Marie and Hargrave (1987) for calculating the distance over which spanner crabs are attracted to bait. This was based on data on the time of arrival of spanner crabs at the bait, speed and direction of water currents at the time of trapping with laboratory-derived data on the response time of the spanner crabs to a food stimulus and the speed at which spanner crabs move over the bottom. 2. Materials and methods 2.1. Field study The study was carried out on the east coast of Queensland to the north and east of Moreton Island
(278100 S, 1538250 E), 1986±1992. Commercial ®shers use a variety of shapes and sizes of tangle nets covered by a range of mesh sizes to catch spanner crabs. We used one of the designs in common use in southern Queensland. This consisted of a circular 1.2 m diameter net having a rim of 12.5 mm diameter steel rod and covered with a single layer of 53 mm stretched mesh net made of 2 mm diameter mono ®lament nylon. One of these nets was fastened across the base of a four-legged frame that supported a television camera 2 m above the bottom. A piece of ®sh (usually Mugil cephalus) was attached to the centre of the net as bait. This is a daytime ®shery so we made all observations in daylight. The camera had a high sensitivity to light and could be used with ambient daylight at all depths down to the maximum worked; this avoided the need for arti®cial light that might disturb the spanner crabs. The wide-angle lens of the camera gave a ®eld of view of approximately 23 m at a distance of 2 m off the seabed. This was suf®cient to show the whole net together with an area outside of the net so that spanner crabs approaching the net could be seen before they encountered the rim. A piece of tufted cord attached near the centre of the net indicated the direction of the water current over the net. A cable carried power to the camera and the television signal to a monitor and recorder in the boat. The frame, with net and camera, was left in place on the bottom for 30 min and the TV signal was recorded at the surface. A Savonius rotor current meter that averaged water speed at 3 min intervals was put onto the bottom about 6 m from the camera each time the system was deployed. 2.2. Analysis of TV information The TV tapes were analysed on a tape recorder allowing a range of playback speeds from single frame operation up to six times normal. The recorder had a digital timer (reading to 1 s) which was used to measure the duration of aspects of the behaviour of spanner crabs. For each spanner crab we recorded the length of time between the start of the experiment (when the net reached the seabed) and the time of arrival of the crab at the rim of the net as well as the direction (in 308 sectors) of arrival. When large numbers arrived at the rim of the net, crabs that were already there sometimes obstructed the new arrivals.
B.J. Hill, T.J. Wassenberg / Fisheries Research 41 (1999) 37±46
These later arrivals occasionally responded by backing away and circling around the net. Some of them disappeared from the ®eld of view and would be counted as new arrivals if they subsequently returned to the net. We compensated for this source of bias by subtracting from the number of arrivals, all spanner crabs that left the ®eld of view without crossing over the rim. This problem arose in sets in which a large number of spanner crabs responded. In one case in which there were 226 arrivals, 34 crabs left the ®eld of view before crossing the rim and we interpreted this set as having attracted 192 individuals. In a set with 125 arrivals, six crabs left the ®eld of view whereas in a set with 35 arrivals, only two left the ®eld of view. The number of crabs that arrived at the net would be biased upwards if crabs that had crossed onto the net and then left it subsequently returned to the net. If a crab crossed over the rim onto the net, we recorded the time of crossing. If it reached the bait we also recorded the time taken to cross the net to the bait and the time spent at the bait. If the crab escaped, we noted the time taken to cross the net from the bait to the rim and the direction of leaving. The direction of the current varied during any particular set and so we noted the direction every three minutes and took the most frequent direction as being the main direction for each set. The direction of arrival and departure of spanner crabs was recorded relative to this main current direction. To avoid problems associated with analysis of angular data (Cain, 1989), we used the V test, as described by Schmidt-Koenig (1975), to test whether the directions of arrival and leaving were uniformly distributed or clustered around the direction of the current. The exact angle of arrival or departure of the crabs was not recorded but only the number in each of 12 sectors. To test whether this method of recording was adequate, the analysis was done twice: 1. assuming that all spanner crabs arrived or departed at the middle of the sector; 2. assuming that the arrivals and departures were uniformly distributed within each sector. The difference in results was negligible indicating that counts in sectors were an adequate way of measuring angle of arrival or departure. We carried out a separate experiment to check whether the size and sex ratio of spanner crabs caught
39
by tangle nets was the same as that of those attracted to bait. The net was removed and replaced by a line attached between diagonally opposite legs of the camera frame. A piece of bait and a 100 mm long plastic rod were tied to the centre of this line. The position of the camera on the frame was lowered to 1 m above the seabed to give a closer view of spanner crabs attracted to the bait. We used sliding callipers to measure the apparent carapace width of crabs on the television monitor screen. This measurement was corrected using a scaling factor obtained by measuring the length of the image of the 100 mm rod. The image on a television monitor is distorted towards the edge and so only crabs at the centre of the monitor were measured. Carapace length rather than width is usually measured in studies of spanner crabs, but when the animals are alive, they often stand with their body angled upward. This would cause a large error in a carapace length measurement viewed from above and so we used carapace width. Individuals with a carapace width larger than about 50 mm can be sexed from the size and shape of the horns on the carapace and we used this characteristic for sexing animals on the video. They were excluded from the analysis if this was not possible because of the angle of the crab or because the carapace was damaged or because the crabs were too small. Each time the camera was lowered in this con®guration, three baited tangle nets were laid at least 100 m from the boat and retrieved after 40±60 min. All spanner crabs caught on these nets were measured and sexed. We used a square root transformation to normalise the data on size composition because it was skewed. We then used an analysis of variance to compare the size and sex of spanner crabs attracted to bait under the camera with those caught on nets at the same time. We used a Chi-Square test to compare the observed number of males and females caught on tangle nets with the expected numbers derived from the experiment in which no net was used. 2.3. Laboratory study The speed at which spanner crabs moved over the bottom was measured in a 12 m long, 1 m wide tank containing 750 mm depth of sea water (temperature 21±238C) and a 150 mm deep layer of sand. Twelve crabs (7 females and 5 males) in the size range 60±
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B.J. Hill, T.J. Wassenberg / Fisheries Research 41 (1999) 37±46
111 mm carapace width, were collected from tangle nets and taken to the laboratory in drums of seawater. The carapace width of each crab was measured; it was labelled by gluing a numbered plastic disc to the carapace and then put into the tank. The crabs were allowed to settle down for two days con®ned to a 1.5 m long section of the tank by a barrier. Following this settling in period, on each of 10 mornings the barrier was lifted and a piece of ®sh was dragged along the length of the tank. Between 1 and 6 crabs (mean 4.2) emerged each day and moved 9 m down the tank after the food stimulus was introduced. The identity number of each crab that responded and the time to travel 9 m along the tank was noted. The crabs were then returned to the space behind the barrier. We used an F-statistic to test whether there was a relationship between speed and carapace width. 2.4. Estimation of distance of attraction We estimated the maximum distance (dmax) from which most spanner crabs could have been attracted to the bait in each set using the relationship dmax t Vcurrent Vcrab = Vcurrent Vcrab ; where dmax is the attraction distance in metres, t the time period in min (30 min in these experiments), Vcurrent the mean speed of current in m minÿ1 and Vcrab is the mean speed of crabs in m minÿ1 (derived experimentally above). We used average spanner crab speeds (as absolute values) in this estimate and so the fastest moving individuals could have come from a greater distance. Fluctuations in current speed could also have spread the bait scent further than is indicated by using average current speeds. 3. Results 3.1. Field study Although the underwater TV system generally worked well, on some occasions, the drag of the current on the power and signal cable caused the camera to fall over. A system using a self-contained camera would remove the need for the cable and probably remedy this problem. We obtained 12 complete and one incomplete 30 min sets, but on two of
these no spanner crabs appeared and so the results from 11 sets in which crabs responded are used in some analysis but only 10 for the numbers arriving and leaving. The harmonic mean current speed was 9.6 cm sÿ1 (5.76 m minÿ1) and ranged between 3.3 and 15.6 cm sÿ1. The current was rarely constant for the 30 min period of a set, the coef®cient of variation ranged from 8% to 62%, but it was mostly between 20% and 40%. Although the current always ran in one main direction, it ¯uctuated around this direction as can be seen in Fig. 1(a) and for 99% of the time it was in a 908 arc. There was a signi®cant negative relationship between the direction from which the crabs arrived and current direction (u22.3, p<0.01) showing that they were moving into the current (Fig. 1(b)). Few crabs arrived in the ®rst 3 min after the net reached the bottom (Table 1). The mean time of ®rst arrival was 7 min 5 s (Table 2) with the shortest time 2 min 21 s. The numbers arriving then rose rapidly to reach a peak between 15 and 21 min after which the rate of arrival declined (Table 2). On an average, 36.7 spanner crabs appeared at the net but the number ranged from 2 to 192 and this wide range resulted in a large standard deviation. As mentioned above, on two occasions no crabs responded. If these two sets are included, then the average number arriving was 31.0. On the set in which 192 crabs appeared, it was not possible to distinguish some aspects of their behaviour because they climbed on top of each other. In this case we recorded the time and direction of arrival of each crab, the number that crossed onto the net as well as
Table 1 The mean numbers (and standard deviation) of spanner crabs (R. ranina) arriving at a baited tangle net in successive three minute periods based on 10 complete 30 min sets Time (min)
Mean
Standard deviation
3 6 9 12 15 18 21 24 27 30
0.3 0.8 2.3 4.1 7.4 6.8 7.3 5.9 5.3 3.7
0.5 1.6 2.8 6.9 13.5 11.8 11.6 9.5 11.2 7.2
B.J. Hill, T.J. Wassenberg / Fisheries Research 41 (1999) 37±46
41
Fig. 1. (a) The percentage frequency of direction in 308 sectors of water currents over the tangle net during field experiments (92 readings). (b) The direction from which spanner crabs arrived at the rim of the net (n404) relative to the direction of the water current over the net. (c) The direction in which spanner crabs left the net (n283 measurements).
the direction and time of departure of those that clearly crossed onto the net. Some crossed the rim of the net immediately, but most stopped at the rim for a short time before cross-
ing (Table 2). The mean time taken to cross the net from the rim to the bait was 22.6 s, but the distribution of times was highly skewed and 51% took 5 s or less. Once they were on the net, progress was often slowed
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Table 2 Results of underwater video observations of the behaviour of spanner crabs (R. ranina) on tangle nets Behaviour
Outcome or sample data
Number of sets Mean time of arrival of first crab at net Total number seen arriving at net Mean number seen arriving at net Percentage crossing onto net Mean time before crossinga Percentage reaching baita Mean time taken to reach baita Mean time at baita Mean time to cross the net after feedinga Percentage of crabs that crossed onto the net and were still there after 30 min
11 sets 368 s (SD165 s) (n10 sets) 404 (n11 sets) 36.7 (SD60.6) (n11 sets) 81% (n404 crabs) 13.0 s (SD24.7) (n172 crabs) 37% (n218 crabs) 22.6 s (SD32.5) (n75 crabs) 113.7 s (SD100.4) (n67 crabs) 48.1 s (SD92.9) (n62 crabs) 7% (n329 crabs)
The value of n varies for different reasons: in one set the first minute of recording was lost and so the time of the first arrival was not obtained; crabs sometimes climbed over each other and so we could not accurately determine when some crossed onto the net; not all spanner crabs that crossed the net managed to reach the bait; not all spanner crabs that reached the bait fed; and not all spanner crabs that left the bait managed to escape from the net. a The set in which 192 crabs arrived at the net is not included in calculations.
because some crabs became entangled or other spanner crabs obstructed them and some never reached the bait. If they did, they fed rapidly, and on average, spent less than 2 min at the bait. Many crabs became temporarily entangled in the mesh while feeding and this slowed their departure. On average it took about twice as long to cross the net from the bait to the rim as it did to cross from the rim to the bait. Crabs that were not entangled walked or swam off the net. They left the net in all directions although fewer did so into the current (Fig. 1(c)). This bias was suf®cient to give a signi®cant relationship (u2.95, p<0.01) between the direction in which the crabs left the net and current direction ± the mean result was that the crabs departed the net in the same direction as the current ¯owed. The low value of u re¯ects the widely dispersed directions taken by departing crabs. Although crabs arrived at the net throughout the period of the set, others were leaving and so the number on the net rose only slowly. Towards the end of the 30 min set, the rate of departure exceeded the rate of arrival and the number on the net dropped (Fig. 2). Only 23 crabs out of the total of 329 that crossed onto the net in the 11 sets were still on it when it was lifted. If it is assumed that all of these would have been caught when the net was lifted, the capture rate would be 7% of those that crossed onto the net and an even lower proportion of those attracted to the bait.
The size of spanner crabs attracted to the bait without a net and the sizes of those caught on adjacent nets set at the same time, are shown in Fig. 3. There is no signi®cant difference between the size of females caught on tangle nets (mean 68.7 mm, SD 7.3, n71) and those attracted to a bait not attached to a net (mean 71.8 mm, SD 8.5, n202) (F2.40). However the F-test indicated that the size of males caught on tangle nets (mean 77.2 mm, SD 12.8, n55) was signi®cantly smaller than those attracted to a bait not attached to a net (mean 81.3 mm, SD 12.1, n142) (F5.08). The difference is minor and given measurement errors and variations in populations, may not be biologically signi®cant. Tangle nets did not show a sex bias: 59% of 344 spanner crabs attracted to a bait were females, compared to 56% of the 126 crabs caught on tangle nets set at the same time. 3.2. Laboratory study Two of the 12 crabs in the tank did not respond to the bait. The remaining 10 walked the full 9 m in the tank at least once during the 10 experiments. Some crabs stopped part way along the tank and either buried, or moved back towards the start. The mean speeds at which the individual spanner crabs travelled are given in Table 3. Speeds varied quite markedly and there was no relationship between speed and carapace width (F0.007, p0.94, n38). We assumed for the
B.J. Hill, T.J. Wassenberg / Fisheries Research 41 (1999) 37±46
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Fig. 2. Cumulative numbers of spanner crabs arriving, leaving and remaining on a baited tangle net. Average figures based on 11 sets.
Table 3 Mean speed of travel of 10 spanner crabs (R. ranina) over 9 m in an experimental tank
Table 4 Estimated maximum distances from which spanner crabs (R. ranina) could have been attracted to a baited net
Carapace width (mm)
Set number
Current speed (m minÿ1)
Distance of attraction (m)
1 3 4 5 6 7 9 10
4.3 4.3 2 5.6 6.3 7.1 8.5 7.9
61 61 40 68 72 75 79 78
n
Speed ÿ1
62 65 70 71 74 81 86 95 105 111 82.0
ÿ1
cm s
m min
4 1 3 3 5 2 7 4 3 5
5.8 4.7 10.1 9.0 7.0 6.8 3.2 7.3 2.2 9.3
3.5 2.8 6.1 5.4 4.2 4.1 1.9 4.4 1.3 5.6
37
Mean 6.4
n is the number of times each individual responded out of 10 trials.
purposes of estimating the distance over which crabs are attracted to bait, that they move at an average speed of 6.4 cm sÿ1, which is equivalent to 3.84 m minÿ1.
Note that the current meter failed on sets is not shown in this list.
3.3. Estimation of distance of attraction The current meter failed on three of the sets giving eight sets with a complete set of current speed data. Using the formula given above and the ®eld and laboratory data, we calculated that in 30 min, spanner crabs could be attracted from a maximum distance of around 40±80 m downstream of the bait (Table 4).
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B.J. Hill, T.J. Wassenberg / Fisheries Research 41 (1999) 37±46
Fig. 3. Size composition of (a) female and (b) male spanner crabs attracted to bait with no net and those caught on a baited tangle net. n202 females at bait without a net, n71 captured on a baited net. n142 males at bait without a net, n55 captured on a baited net.
4. Discussion In interpreting our results, we have assumed a scenario in which on each occasion a net is set, a number of R. ranina in the area will be exposed to the bait scent. Some of these respond to the scent and move up current to reach the net within the soak time. These crabs are available to be caught but will be captured only if they cross onto the net and become entangled. The process starts when the net reaches the bottom. The mean time taken for the ®rst spanner crab to arrive at the net was 7 min 5 s after the net arrived
on the seabed. This delay may be due to crabs that were very close to the bait being inhibited from responding rapidly because of disturbances such as the boat overhead and the arrival of the camera frame and net on the bottom. Some of the delay is probably a function of the time taken by the crabs to respond to a bait scent. Skinner and Hill (1987) found that on an average 37% of spanner crabs responded within 4.5 min of being exposed to a bait scent. The number of spanner crabs arriving at the net rose to a peak between 12 and 21 min after the start. After this, it fell presumably because crabs coming to the bait are
B.J. Hill, T.J. Wassenberg / Fisheries Research 41 (1999) 37±46
depleting the subset of the population in the area exposed to the bait scent and so there are progressively fewer available to respond. Leaching of the bait is probably not a factor in this ®shery because of the short soak time (see Zimmer-Faust and Case, 1983 for a discussion on bait leaching). Assuming the average speed of spanner crabs when moving towards a food sources is 3.8 m minÿ1, they could be attracted from up to 69 m away in a 30 min set. However, our laboratory experiments showed that some crabs can move at speeds up to 6.1 m minÿ1 and so some spanner crabs might be attracted from further than suggested by the average speed. Sinoda and Kobayasi (1969) assumed that the area ®shed by a baited trap for spider crabs was circular. This is unlikely to apply to situations in which there is a current. Skinner and Hill (1987) found that R. ranina responded to the presence of food by emerging from the sand and moving up current. In the present study, spanner crabs arriving at the net were moving into the current. The effective distance the bait scent travels is obviously related to its concentration and the current velocity. Our derived data suggests a direct correlation between current velocity and distance of attraction but this needs further investigation. We found that for about 60% of the time in our study area, the current direction was from one 308 arc, and for nearly all of the rest of the time it came from adjacent 308 arcs. Under these conditions, most of the bait odour would spread out in a triangular plume subtending a 908 angle from the bait and this is probably the approximate shape of the area of attraction of the bait. Experimentally, not all crabs responded to bait. This also likely to apply in the ®eld. Thus not all spanner crabs around a net are immediately vulnerable to capture, only those down current of it. Obviously this could change suddenly with a change in direction of water ¯ow. The level of escape from the design of tangle net that we used appears to be very high. On an average only 7% of crabs that crossed onto the net were still there after 30 min. In the case when 192 individuals were recorded as arriving at the net, 139 crossed over the rim, but 20 was the highest number of crabs on the net at any one time and only 15 were still on the net after 30 min ± 7.8% of the arrivals. The low retention rate may have been due to the way the net was hung, we noticed that the mesh tended to become smothered
45
in sand ± often because of crabs attempting to bury while on the net ± and this reduced the probability of entangling other crabs. Nevertheless, on an average there were only nine crabs on the net after 30 min. This is close to the catch rate of 10 spanner crabs per hour reported by Kennelly (1989) and so our tangle net was catching at a similar rate. The proportion that is captured could be increased if more spanner crabs could be prevented from escaping. This can be achieved by having a double layer of mesh (Kennelly and Craig, 1989). However that would make it more dif®cult to disentangle the crabs and increases the likelihood that crabs discarded because they are undersized or ovigerous, will lose limbs in the process. Kennelly et al. (1990) found signi®cant mortalities in spanner crabs that had lost limbs. This has been con®rmed by Kirkwood and Brown (1998) who reported varying mortalities depending on the extent of damage. Removal of a single dactylus resulted in 20% mortality while removal of a single cheliped led to 90% mortality. Consequently, doubling of the mesh is not favoured as a strategy. We conclude that the equipment used in this ®shery is not ef®cient. Given the increasing ®shing pressure being exerted on stocks, development of a more ef®cient method of capturing the spanner crabs may be inappropriate. However, a catching method that eliminates limb damage is needed urgently. Acknowledgements The Queensland Department of Primary Industries (QDPI) assisted in this study by making a research vessel available for some of the ®eldwork. We also wish to thank Dr. Ian Brown of QDPI for fruitful discussions about the project, assistance in ®eldwork and valuable comments on the paper. Dr. YouGan Wang of the CSIRO Biometrics Unit kindly provided assistance with the analysis of the angular data and Dr. David Die made useful comments about the paper. References Bourne, G.C., 1922. A study in carcinology. J. Linn. Soc. (Zool) 35, 25±78. Cain, M.L., 1989. The analysis of angular data in ecological field studies. Ecol. 70, 1540±1543.
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John, J. (Ed.), 1994. Managing redundancy in over exploited fisheries. World Bank Discussion papers; 240 III Series: Fisheries Series 128. Kennelly, S.J., 1989. Effects of soak-time and spatial heterogeneity on sampling populations of spanner crabs Ranina ranina. Mar. Ecol. Prog. Ser. 55, 141±147. Kennelly, S.J., 1992. Distributions, abundances and current status of exploited populations of spanner crabs Ranina ranina off the east coast of Australia. Mar. Ecol. Prog. Ser. 85, 227±235. Kennelly, S.J., Craig, J.R., 1989. Effects of trap design, independence of traps and bait on sampling populations of spanner crabs Ranina ranina. Mar. Ecol. Prog. Ser. 51, 49±56. Kennelly, S.J., Watkins, D., 1994. Fecundity and reproductive period, and their relationship to catch rates of spanner crabs, Ranina ranina, off the east coast of Australia. J. Crust. Biol. 14, 146±150. Kennelly, S.J., Watkins, D., Craig, J.R., 1990. Mortality of discarded spanner crabs Ranina ranina (Linnaeus) in a tangle net fishery ± laboratory and field experiments. J. Exp. Mar. Biol. Ecol. 140, 39±48. Kirkwood, J.M., Brown, I.W., 1998. Effect of limb damage on the survival and burial time of discarded spanner crabs Ranina ranina (Linnaeus). Mar. Freshwater Res. 49, 41±45.
Krouse J.S., 1989. Performance and selectivity of trap fisheries for crustaceans. In: Caddy, J.F. (Ed.), Marine Invertebrate Fisheries: Their Assessment and Management. Wiley, New York, pp. 327±374. Sainte-Marie, B., Hargrave, B.T., 1987. Estimation of scavenger abundance and distance of attraction to bait. Mar. Biol. 94, 431±443. Schmidt-Koenig, K., 1975. Migration and homing in animals. Zoophys and Ecol, vol. 6. Springer, Berlin, pp. 1±99. Sinoda, M., Kobayasi, T., 1969. Studies of the fishery of Zuwai crab in the Japan Sea VI. Efficiency of the toyama kago (a kind of crab trap) in capturing beni-zuwai crab. Bull. Jpn. Soc. Sci. Fish 35, 948±956. Skinner, D.G., Hill, B.J., 1987. Feeding and reproductive behaviour and their effect on catchability of the spanner crab Ranina ranina. Mar. Biol. 94, 211±218. Zimmer-Faust, R.K., Case, J.F., 1983. A proposed dual role of odour in the foraging by the California spiny lobster, Panulirus interruptus (Randall). Biol. Bull. Mar. Biol. Lab Woods Hole 164, 341±353.