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Robert J. Miller, Saturation of crab traps: reduced entry and escapement, ICES Journal of Marine Science, Volume 38, Issue 3, August 1979, Pages 338–345, https://doi.org/10.1093/icesjms/38.3.338
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Abstract
Trap saturation, observed as a reduction in catch rate with increasing catch, has an important limitation on catches of Cancer productus and C. magister. However, the gap between potential and realized catch/trap-day could be reduced by modifying trap design and fishing strategy. The size of this gap was estimated in fishing experiments on natural crab populations by comparing catches in traps of the same design and fished in the same way except that in fished treatments crabs were removed from traps every 2 h for 12 h, and in not-fished treatments crabs were merely counted every 2 h for 12 h, but not removed. The cumulative catch from the fished treatment provided a minimum estimate of the crabs available to be caught without the catch limitation of saturation. Regardless of trap type, by 8 h elapsed time cumulative catches in fished traps were always significantly higher than observed catches in traps not-fished. By 12 h elapsed time the former was at least 1–7 times the latter in all experiments.
While saturation could not be eliminated, the catch level at saturation was increased. After 12 h the catch ratios in three trap sizes were 1·0:4–6:9·4 while the ratio of trap sizes was only 1·0:2·0:2:7. Thus, the packing density was higher in larger traps. This was partly explained by different rates of escape from traps. Preventing escapement increased catches by 1–6 times over 12 h. Catches in traps with bait exposed was 2–2 times higher after 12 h than in traps with bait enclosed in perforated containers.
Saturation was shown to have components of escapement and reduced entry since the saturation level increased when escape was prevented, yet the cumulative catch in fished traps of the same type was still higher. Reduced entry was assumed to be due to crabs inside traps intimidating those outside which otherwise would have entered.