Morphological and diet variation in Chrysophrys auratus populations

Abstract

Unrecognised intraspecific variation within a fishery can have sustainability consequences. Chrysophrys auratus (snapper) are an important species for many New Zealand stakeholders, but there is evidence that management units don’t line up with biological populations around the country, which could result in a loss of intraspecific biodiversity and localised depletions. This thesis aimed to better understand the biological population structure and intraspecific biodiversity of C. auratus to help to conserve the species in the face of fishing pressure and better understand how they may respond to environmental changes. To do this, geometric morphometric techniques were used on photographs of 329 New Zealand and 79 Australian C. auratus. Otolith morphology measurements were also used for population delineation. Jaw morphology and diet analysis of the stomach contents were then used to understand the functional feeding morphology of C. auratus. Significant differences between populations in external body morphology were observed, with differences most pronounced in the head curvature, body depth, eye size and caudal peduncle width. Otolith morphology was a less successful technique for population delineation than external morphology in C. auratus, but there were still significant differences in otolith shape between populations. C. auratus consumed a diverse diet spanning many functional groups, but crustaceans, polychaetes, echinoderms, molluscs and teleosts made up most of the diet. Proportions of different prey varied by region, with a more pelagic diet recorded in the Hauraki Gulf and Bay of Plenty populations and a harder, more crustacean dominant diet in East Northland and on the East Coast of the North Island around Gisborne and Hawkes Bay. The hardness of the diet was weakly linked to jaw and head morphology, especially the jaw and tooth width variables, which determine the crushing strength of a fish’s jaw. The generalist nature of C. auratus meant that any jaw adaptations still enabled the consumption of a variety of prey. The polymorphism and functional morphology identified in C. auratus has implications for fisheries compliance, ecological effects of population recovery and how the species will respond to environmental change. The ecomorphological insight should be utilised with appropriate fisheries management units to preserve their valuable, intraspecific variation.