Diet and foraging behaviour of juvenile rig (Mustelus lenticulatus) from New Zealand harbours and estuaries : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Conservation Biology at Massey University, Manawatu, New Zealand
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Date
2012
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Massey University
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Abstract
Smooth-hounds (Elasmobranchii, Triakidae) can form important commercial fisheries,
and in New Zealand, rig (Mustelus lenticulatus) is marketed as “lemonfish”. Despite
this, little is known of their biology. Rig are small sharks known for making annual
inshore migrations to harbours and estuaries to give birth and mate. These areas act as
nursery grounds for newborn rig, providing an important food source, protection from
predators, or both. A large-scale survey of the diet of juvenile (<1 year old) rig was
undertaken throughout New Zealand in February-March 2011, sampling guts of 130 rig
at eight sites from the northern North Island to the southern South Island. Rig fed
mainly on benthic crustaceans, especially stalk-eyed mud crabs (Hemiplax hirtipes) and
snapping shrimp (Alpheus richardsoni). Other prey groups found in their diet include
mantis shrimps, hermit crabs, squat lobsters, various caridean shrimps and
polychaetes, while molluscs were rarely taken and fish were not found at all. Two
recently introduced species were found in rig diets from northern sites: the Japanese
mantis shrimp (Oratosquilla oratoria) from Kaipara Harbour and the greentail prawn
(Metapenaeus bennettae). While diets overlapped between all harbours and estuaries,
significant differences were detected through pairwise Analyses of Similarity between
sites. Differences in diet were associated with latitude and temperature, and related
especially to the proportions of two mud crabs, Hemiplax hirtipes and Hemigrapsus
crenulatus, the snapping shrimp Alpheus richarsoni and the prawn Metapenaeus
bennettae. We suggest that newborn rig remain in harbours and estuaries primarily to
feed. In addition to analysing juvenile rig diet, a behaviour study was performed to
analyse the effects of sediment type on captive juvenile rig foraging effort and success.
Six young of the year rig caught from Porirua Harbour were transferred to the NIWA,
Greta Point, Wellington facility. No significant differences were observed in the time
spent foraging or the number of strikes occurring on sand or mud. However, a
significant increase in the time spent foraging and a significant decrease in the time
spent resting was observed with the presence of crabs. Further research is required to
determine the effects of sedimentation on juvenile rig behaviour.
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Keywords
Mustelus lenticulatus, Smooth-hound shark, Lemonfish, Rig shark, Rig diet, Rig behaviour