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Fishery Bulletin 117(3) 
traps are covered or disabled. As a result, the number of 
lobsters captured per unit of time by these traps can be 
significantly greater than standard traps, and the size 
distribution more accurately represents the population 
on the seafloor (Courchene and Stokesbury, 2011). This 
leads to a better relationship between catch in ventless 
traps and the density of lobsters on the bottom than with 
standard, vented traps (Watson and Jury, 2013). Neverthe¬ 
less, we recently demonstrated that the catch in ventless 
traps reaches a plateau, or “saturates,” after soak times 
of <24 h, even though survey soak times are typically 
>3 days (Clark et al., 2015). This could influence estimates 
of lobster abundance, especially in areas where the den¬ 
sity is high. In their 2015 report, in recognition of this 
concern, the Atlantic States Marine Fisheries Commission 
(ASMFC) stated that it was a high priority to conduct fur¬ 
ther research to help “calibrate” the relationship between 
ventless trap catch and the density of lobsters on the bot¬ 
tom because “ventless traps may be limited in their ability 
to differentiate between moderately high and extremely 
high abundance” (ASMFC, 2015). 
Trap saturation, defined by Miller (1979) as a decrease 
in catch rate with increasing numbers of lobsters in a trap, 
is a common phenomenon for many types of traps used 
in a variety of fish and crustacean fisheries (Miller, 1990; 
Fogarty and Addison, 1997; Stoner, 2004; Hedgarde et al., 
2018). In one of the first studies addressing this phenom¬ 
enon, the “saturation effect” was observed in squirrelfish 
(.Holocentrus adscensionis) and sablefish (Anoplopoma 
fimbria) pots, and it appeared to be due to a decrease 
in entry rate as soak time increased (High and Beards¬ 
ley, 1970). In 1985, Auster demonstrated the asymptotic 
nature of catch of American lobsters in standard traps 
over 6-7-day soaks. In 1996, Miller and Rodger reported 
that standard lobster traps saturated within 12 h of being 
deployed. Furthermore, Fogarty and Addison (1997) mod¬ 
eled the effects of multiple variables on standard trap 
saturation and these models, which included variables 
associated with entry rate, escape rate, and changes in 
these variables over time, yielded data that compared 
favorably with Auster’s (1985) data. More recently, using a 
trap-mounted time-lapse video system, we demonstrated 
that ventless traps saturate in <24 h because they reach 
a dynamic equilibrium where entry and escape rates are 
equivalent (Clark et al., 2018). Because the escape rate is 
low in ventless traps, variability in entry rate is likely a 
key factor that leads to trap saturation. 
Mechanisms that might be responsible for ventless lob¬ 
ster trap saturation, or a plateau in catch after a certain 
period of time, include 1) deterioration of the bait and/or a 
decrease in its attractiveness; 2) removal/capture of most of 
the animals in the area fished; and 3) interactions between 
lobsters in, and around, the trap. The distance of bait attrac¬ 
tion for lobsters to standard traps has been previously 
estimated as approximately 11 m from the odorant source 
(Watson et al, 2009), and we expect that ventless traps fish 
similarly in terms of bait attraction. While the fishable area 
of a trap will depend upon habitat conditions (e.g., current, 
temperature, bottom type), lobsters that are already in, or 
move into, the area of bait attraction will likely to be drawn 
to the trap. However, as bait is removed by the feeding activ¬ 
ity of lobsters and other species, it may deteriorate and this 
will lead, in part, to a decline in the release of amino acids 
(and other potential attractants) from the bait over time 
(Maekie et al., 1980; Lpkkeborg, 1990; Kamio and Derby, 
2017). Moreover, as bait quality deteriorates, the catch rate 
of lobsters is also expected to decline because the area of 
attraction will be reduced, fewer lobsters will be attracted 
to the trap, and entry rate will decrease. 
Although loss of bait attractiveness is a likely factor 
affecting trap saturation in both standard and ventless 
traps, it is possible that, at least for venfless traps, cap¬ 
ture and retention of many of the lobsters in the effec¬ 
tive fishing area (EFA) (Miller, 1975) is another factor. 
Some of the single-parlor ventless traps fished by the 
Massachusetts Division of Marine Fisheries have been 
shown to retain up to 50 lobsters (MABMF 2 ). Therefore,' 
depending on the initial density of lobsters on the bottom, 
after a 24-h soak most of the lobsters within the fishing 
area of the trap might be retained in the trap, and so 
there would be few lobsters remaining in the vicinity 
that could be captured. As a result, approach and entry 
rates would be reduced, catch rate would plateau, and 
traps would become “saturated.” 
Standard and ventless trap saturation is also likely a 
function of behavioral interactions, with animals in the trap 
inhibiting the entry of subsequent animals, as Barber and 
Cobb (2009) demonstrated with Dungeness crabs ( Cancer 
magister). Similar interactions have been observed in and 
around American lobster traps (Richards et al., 1983; Jury 
et al., 2001; Watson and Jury, 2013), blue crab ( Callinectes 
sapidus ) traps (Sturdivant and Clark, 2011), and cod pots 
(Anders et al., 2017). It has been proposed that, as traps 
fill, these antagonistic interactions increase, reducing the 
further entry of lobsters and increasing the likelihood that 
lobsters will escape. This phenomenon might be even more 
pronounced for ventless traps, given their overall greater 
tendency to rapidly fill with lobsters. 
The overall goal of this study was to test 3 possible mech¬ 
anisms underlying saturation of ventless lobster traps: 
1) so many of the lobsters in the fishable area of a trap get 
captured after 24 h that entry and exit rates equalize and 
subsequent catch plateaus; 2) catch levels off because the 
bait loses its attractiveness; and 3) the presence of lobsters 
in the trap inhibits the entry of additional lobsters. 
Materials and methods 
Overview 
We conducted the following 4 types of experiments to test 
the hypotheses stated above: 1) quantifying the number 
of lobsters in the vicinity of ventless lobster traps before, 
2 MABMF (Massachusetts Division of Marine Fisheries). 2017. 
Unpubl. data. Mass. Div. Mar. Fish., 251 Causeway St., Ste. 400, 
Boston, MA 02114.] 
