Feeding ecology of rig (Mustelus lenticulatus Phillipps, 1932: Elasmobranchii) from Golden Bay - North West Nelson in relation to seasonal movement pattern, reproduction and condition

Abstract

The food and feeding of rig (Mustelus lenticulatus) is examined from Golden Bay, North-west Nelson with respect to movement pattern, reproduction and condition. Movement pattern and reproduction are determined from records and observations of rig capture position, sex ratios and reproductive states of catches from commercially operated setnets. Rig condition is evaluated in terms of relative liver weights expressed as a percentage of body weight, body weight being calculated from length-weight equations. Weight of stomach contents expressed as a percentage of body weight, and mean prey numbers are used to describe the intensity of feeding. The composition of prey in rig diets is described by the percentage "index of relative importance" (IRI). Rig diets are briefly examined from other locations around New Zealand by volumetric proportions estimated subjectively. To support field data, observations and experiments were performed on rig in captivity. In Golden Bay rig form schools of individuals of similar size and sex. They tend to occupy the benthic zone to at least eight metres from the bottom, with males occupying deeper regions from October until December and females occupying shallower water from December until March. In Golden Bay embryos up to 15 cm TL are found. There is some evidence to suggest that parturition and copulation take place outside of Golden Bay and that juveniles occupy the Bay for the entire year. The hypothesis that the inshore movement of rig is a feeding migration in which females gain condition between pregnancies could not be substantiated, females being found to lose condition rapidly in shallow water. Alternative hypotheses are that rig move inshore to locate mates and to confer advantage on juveniles by providing an adequate food supply and protection from predation. The persistence of females in shallow water while rapidly losing condition could not be explained. The hypothesis that the level of liver lipid reserves changes with rig length as a response to the changing metabolic demands of growth and reproduction could not be substantiated. In males, monthly changes of condition were not consistent with the predicted period of copulation. Stationary or slow moving macro-benthic invertebrates are common dietary components of all members of the genus Mustelus, but some species of Mustelus also include teleosts and fast moving cephalopods as major components of their diets. Rig feed opportunistically within the benthic zone, consuming almost exclusively slow moving or stationary macro-benthic invertebrates, particularly crustaceans. The ecological prey type does not change between locations around New Zealand. The size range of prey increases with size of rig. Rig are found to feed most intensively at night. They require from three to six days to evacuate a meal. Rig show litle size selectivity, this being attributed to the minor role that the visual sense is involved in prey location. In Golden Bay males tend to feed on the echiurid Urechis novaezalandiae and females tend to feed on the brachyurans Nectocarcinus antarcticus, Cancer novaezelandiae and Ovalipes catharus. Pagurids and polychaetes are common prey of both sexes. Although food species differs between deep and shallow water habitats each sex consumes the same ecological prey type and in approximately the same quantity.