722 



Fishery Bulletin 90(4), 1992 



of lobsters prior to their deployment in traps was determined 

 using Lyle's (1982) adaptation of Drach's (1939) staging 

 technique. 



In the field test, 128 tagged lobsters were placed by divers 

 in 4 strings of 8 unbaited traps each (2 spiny and 2 slipper 

 lobsters per trap). Two strings were placed on and around 

 high-relief substrate, one in summer 1990 and the other in 

 summer 1991; two strings were placed on relatively-flat 

 hardbottom, at least 300 m from any relief that could pro- 

 vide lobsters shelter, in summer 1991. The contents of the 

 traps were checked every 48 hours until all tagged lobsters 

 had exited or died. 



In the laboratory trials, 64 additional tagged lobsters 

 were placed in 16 traps (2 spiny and 2 slipper lobsters per 

 trap) in a large, shaded, outdoor concrete tank. Through- 

 out the tank, food and other suitable shelter were provided 

 outside the traps to encourage exiting. Contents of the traps 

 were monitored daily, and any lobsters found outside the 

 trap in which they were originally stocked were removed 

 from the tank. Lobster death totals in the laboratory (where 

 predation could not occur) and in the field were compared 

 in an attempt to separate losses by predation from other 

 mortality (e.g., starvation, conspecific aggression) in the 

 field. 



This study employed a modified experimental cohort de- 

 sign to examine the effects of multiple categories (replicate, 

 habitat type, species) on exiting by lobsters. The design 

 permits cross-classified categorical analysis to be applied 

 (Fienberg 1987). Using chi-square tests, comparisons were 

 made between replicates, habitat types, and species in a 

 systematic order. Categories were collapsed or pooled when 

 justified by the lack of significant differences (Siegel and 

 Castellan 1988). 



Results 



Trap stability and faunal interactions 



Estimated seas of 4-6 ft and currents of 1-2 knots were com- 

 mon at the study site. They produced no observable effect 

 on the physical integrity of the plastic traps. Movement of 

 traps across the substrate was not detected, despite frequent 

 observations of the interconnecting groundline actively mov- 

 ing in the bottom surge. The two halves of the trap without 

 hinge pins shifted 2cm apart. Over the 6-month period, the 

 traps became encrusted with sessile organisms, including 

 bryozoans, corals, and fish eggs. Occasionally adult fish 

 larger than the opening of the escape vent were found in 

 the traps; however, most of these departed through the trap 

 entrance as a diver approached. 



Adults of both spiny and slipper lobsters local to the sur- 

 rounding study site entered the traps. Of the 12 such occur- 

 rences of lobsters recorded during the 6-month survey, 7 

 lobsters left before the last inspection of the monthly obser- 

 vation period, indicating that they did not occupy the traps 



for more than 30 consecutive days. Three lobsters 

 were observed entering and exiting within the 

 same 48-hour observation period. Nine of the 12 

 lobsters were found in traps on even bottom. 



One dead spiny lobster comprised the only mor- 

 tality observed within the 6-month field evalua- 

 tion. Postmortem examination and the presence 

 of lobster debris in the area around the trap sug- 

 gested death by predation. Sightings of known 

 predators such as octopus, eels, jacks, and sharks 

 were routine. Large eels often occupied the traps, 

 occasionally with lobsters. 



Trap stocl<ing experiment 



Molt-stage evaluation indicated that 27% of the 

 spiny lobsters and 1% of the slipper lobsters were 

 in the premolt stage at deployment. Mortality was 

 limited to seven spiny lobsters, five in the labor- 

 atory and two in the field. All of these lobsters 

 were in premolt stage at deployment and died 

 during or shortly after molting. Despite the 

 limited mortality, the number of deaths in the 

 laboratory trials differed significantly from zero 



c 



_o 



C 

 O 



o 



ID 



O 







-1 



2 

 3 - 



■4 

 ■5 -- 

 6 



N = 126 a 



H 





1 1 1 1 1 1 1 i' I 



I ' I ' 1 1 



Days 



Figure 2 



Persistence of occupancy of plastic traps stocked with 

 spiny Panulirus marginatus, and slipper Syllarides 

 squamosiis, lobsters (connbined) in the (a) field and (b) 

 laboratory. 



