Catchability

Estimation of crab abundance based on a feeding response, requires an understanding of all the processes involved in the capture process. Two key factors influence the capture of crustaceans in baited traps: the relationship between animal abundance and the effectiveness of the fishing gear (commonly termed catchability), and the mosaic of physiological, environmental and behavioural interactions that affect the species of interest.

The catch of a baited trap is the result of a series of interactions between the animal’s attraction to the bait, the environment, and the trap (Bennett 1974 in Robertson 1989)."Trap catches are, therefore, affected by many factors, both biotic (e.g. physiological and reproductive condition of the target species, feeding rhythms, intraspecific attraction or competition) and abiotic (e.g. temperature, tidal cycle, shape and size of the entrance to the traps)" (Robertson 1989).

Williams and Hill (1982) identify two important factors affecting the feeding activity of decapod crustaceans; the physiological state of the animal (i.e. the moult stage) and, the environmental conditions, such as water temperature. Activity and appetite of decapods (and numerous other species) often increases with temperature and bait attractiveness also increases due to the rate of diffusion of bait molecules increasing with temperature (Morrissy 1975).

Traps provide a passive method of capture where capture success is acutely linked to the behaviour of the target species (Fogarty and Addison 1997) and the ability of the gear to retain captured animals.

Arreguin-Sanchez (1996) defines catchability as a measure of the interaction between the resource abundance and the fishing effort. The author identifies the two key parameters of interest as "gaining a measure of the fishing gear efficiency (selectivity) and the relationship between population size and fishing effort".

It is not unusual for animals to demonstrate some form of natural response or learned behaviour when confronted with a baited trap. Some animals become "trap-happy" while others will not be observed again and are termed "trap-shy". Summerlin and Wolfe (1973) in studying the patterns of trapping behaviour of the cotton rat, Sigmodon hispidus found a distinct pattern of dominant animals inhibiting capture of subordinate animals. This behaviour diminished as dominants were removed. Miller (1990) suggests that aggressive behaviour of large lobsters and crabs may also deter small animals from entering a trap. He noted frequent agnostic encounters between crabs (Cancer productus, C. irroratus and Hyas araneus) inside and outside traps and suggests that this behaviour may reduce entry of more crabs to a trap. The author also notes laboratory studies on several species of decapods demonstrating that larger conspecifics usually win agonistic encounters and it is only when the size differential is at its greatest that conspecifics appear to ignore one another (Schrivener 1971 in Miller 1990). Chittleborough (1974) studied the associated trapping behaviour of Western rock lobster (Panulirus longipes cygnus) and found that while younger individuals were abundant in the

study site, competition for food resulted in a dominance of the larger juveniles preventing the smaller lobsters from entering baited traps.

Robertson (1989) suggests that large male S. serrata display intimidatory behaviour which may reduce entry to a trap and encourage escape. Observations from the field program described in this thesis suggest that large S. serrata initially dominate trap catches and it is only when large animals are removed that smaller individuals become more abundant in the catch. This is similar to the observations of Frusher and Hoenig (2001) who found that removal of large southern rock lobsters (Jasus edwardsii) from a Tasmanian population by trapping resulted in increased catchability of smaller lobsters, demonstrating a size related dominance hierarchy affecting trap selectivity.

5.1.1 Commercial Catch Variation

Commercial catch and effort data for the Northern Territory mud crab fishery has been collected since 1983 (Figure 3). High levels of variation in catch are apparent and season factors appear to influence catch and catch rates. Mean catch and effort data for the period 1999 to 2003 demonstrates a seasonal trend with 67% the total catch harvested during the six month dry season period (May-October) (Figure 12). This seasonal variation may be partially explained by key biological and environmental factors, such as reproduction, growth and the onset of the monsoon. Variation in levels of fishing effort is also apparent with an average of 58% of total effort applied during the dry season. Unfortunately due to remoteness of the fishing grounds, reported fishing effort is difficult to validate and use of excess traps is the most common offence in remote regions of the fishery. Fishery related factors may also impact on the seasonal nature of this fishery such as logistical problems that occur when operating from remote locations during the monsoon season (Nov-April). This includes access to fuel during the wet season. More recently native title claims excluding access to the inter- tidal zone have been lodged and granted for some areas.

Figure 12. Mean monthly catch in tonnes (closed histogram), effort traplifts/1000 (open histogram) and CPUE kg per traplift (open circle) for the Northern Territory Mud Crab Fishery 1999 to 2003.

0 20 40 60 80 100

jan feb mar apr may jun jul aug sep oct nov dec

A v c a tc h (t) & e ffo rt (tr a p li fts /1 0 0 0 ) 0.00 0.20 0.40 0.60 0.80 1.00 C P U E (k g /tr a p li ft)