Clark et al.: Underwater video surveillance of catch saturation in lobster traps 
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40 
20 
Ventless 
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9 10 15 16 17 
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Accumulated entries 
-o- Accumulated escapes 
-x- Catch 
Number of entries, escapes, and catch of American 
lobsters (Homarus americanus) accumulated per hour 
during a single representative trial with a ventless 
trap and a single trial with a standard trap over 8 h 
on day 1 and 5-7 h on day 2 (no data were collected at 
night). Both traps were deployed off New Hampshire 
(Wallis Sands) during 2010-2012. 
For standard traps, approximately the same number 
of lobsters entered (Mann-Whitney (7=6.5, P>0.05), es¬ 
caped (Mann-Whitney (7=7, P>0.05), and surrounded 
the traps (Mann-Whitney U-5, P> 0.05) throughout the 
2-d soak. 
Comparison of half-entries on days 1 and 2 of the soak period 
A similar number of half-entries took place in the 2 
types of traps on day 1 (Fig. 7; Mann-Whitney (7=5, 
P>0.05) and on day 2 (Mann-Whitney (7=4.5, P>0.05). 
Moreover, there was a higher frequency of half-entries 
on day 1 than on day 2 for standard and ventless traps 
(Mann-Whitney U- 9, P< 0.05). 
While quantifying half-entries, we identified the fol¬ 
lowing types of deterrents to entrapping American lob¬ 
sters: 1) disturbance of entering lobsters by approach¬ 
ing conspecifics; 2) agonistic interactions of approach¬ 
ing lobsters with lobsters that were already inside the 
trap; 3) current-induced movements of the bait bag 
that potentially startled lobsters; and 4) apparent loss 
of interest of approaching lobsters (cause unknown). 
Deterrents for half-entries were classified as unknown 
if a lobster approached the trap, usually making con¬ 
tact with it, but left without any obvious cause. Un¬ 
like lobsters that exhibited a half-entry for one of the 
reasons listed above, the lobsters that half-entered 
for no apparent reason rarely exhibited an avoidance 
response (i.e., tail-flipping) before leaving the field of 
view. In both types of traps, half-entries occurred pre¬ 
dominantly because of either loss of interest (unknown) 
or intimidation by approaching lobsters. Loss of inter¬ 
est was responsible for 43% and 55% of the half-entries 
observed in ventless and standard traps, respectively, 
and intimidation by approaching lobsters accounted 
for almost 30% of the half-entries in each trap type. 
Agonistic interactions with lobsters inside of ventless 
and standard traps were responsible for 18% and 11% 
of the half-entries, respectively. The movement of bait 
bags within each trap had the least effect, causing 
<10% of the half-entries. 
Accumulated entries and escapes 
Some trap dynamics differed between day 1 and day 
2. For ventless traps, the rates of entry, escape, and 
catch on day 1 exceeded those observed on day 2 (Figs. 
5 and 6; Mann-Whitney (7=9, P<0.05 for all variables). 
For standard traps, the rates of entry, escape, and 
catch did not significantly differ over the 2-d soak 
(Mann-Whitney (7=7, P>0.05). Because entry rate and 
exit rate were equivalent on each day, standard traps 
caught lobsters at similarly low rates on days 1 and 
2 (Mann-Whitney (7=6, P>0.05); therefore, very few, 
if any, lobsters were captured by the standard traps. 
In contrast, ventless traps had higher CPUE on day 1 
than on day 2 (Mann-Whitney (7=9, P<0.05). The lob¬ 
sters could not easily leave the ventless traps because 
of the absence of an escape vent and, as a result, es¬ 
cape rates were lower than entry rates on day 1 (~1 
individual/h in contrast with 7 individuals/h, respec¬ 
tively). Ventless traps continued to accumulate lobsters 
until catch plateaued on day 2 (Fig. 5). At the end of 
each trial, ventless traps had higher CPUE than stan¬ 
dard traps (Fig. 7; Mann-Whitney (7=9, P<0.05). 
Discussion 
The results from this set of experiments provide in¬ 
sight into the mechanisms that are likely responsible 
for the saturation of lobster traps, particularly ventless 
traps. For example, catch of American lobsters in vent¬ 
less traps reached a plateau on day 2 of a soak owing, 
in part, to a reduction in the rate at which lobsters 
entered the traps. The data illustrated in Figures 3 
and 5 indicate that this leveling off of entry rate may 
be due, at least partly, to a decrease in the density 
of lobsters in the vicinity of the trap. This decrease 
in the number of lobsters surrounding traps may have 
occurred because ventless traps accumulated so many 
lobsters that they effectively reduced the local density 
of lobsters. However, in a more recent study (W. Wat¬ 
son, unpubl. data), the results of dive surveys that cor¬ 
responded with trap trials indicate that the number of 
lobsters in the fishable area did not change between 
day 1 and day 2; therefore, we hypothesize that other 
factors, such as loss of bait attractiveness after approx¬ 
imately 24 h, may be the reason that fewer lobsters 
approached and entered both types of traps on day 2 
versus day 1. 
