Aspects of the fisheries ecology of Pecten novaezelandiae stocks in northern New Zealand. Redefining parameter estimation and modelling techniques

Abstract

The main objective of this research was to advance our ability to devise an assessment model which can be specifically applied to Pecten novaezelandiae scallop stocks in future. This was achieved via collation and re-evaluation of survey data records to determine the appropriate scale of assessment and provide a biomass index. A delay difference modelling approach was applied to the data to estimate biomass over time and to understand exploitation and survival within the most prevalent statistical fishing areas within the Coromandel and Northland fisheries. In addition, biological studies examined spat growth over time using a chemical marker, calcein and natural mortality via adult shell degradation and biofouling experiments to approximate time of death. Condition indices from a number of sampled scallops were also evaluated to gain information on their growth potential. Clucker studies estimated an average shell separation time of 320 days and biofouling timelines were classified, with valves separating in an exponential fashion. Calcein was a suitable marker, with a strong mark produced and no negative effects on the animal’s growth, condition or survival resulting from this treatment. Growth measures and aging methods were successfully applied, with an average growth rate of approximately 0.036 mm per day corresponding to an average striae production per day of 0.53. A condition index utilising a length ~ meat weight relationship was applied and utilised to investigate differences in condition between scallops collected in a number of locations and over a number of years, with areas of high exposure and low sedimentation yielding higher condition indices. Such information is applicable to potential growth estimates in different locations and environmental conditions, as condition is a measure of growth potential. Dredge efficiency corrections gave new biomass estimates to historical survey data, influencing predicted changes over time. The delay difference modelling technique was applied successfully in 3 locations where sufficient survey data were available. The delay difference model fitted the survey estimates well, and provided useful information on past mortality events unlikely due to fishing, which were further investigated. Such stock assessment modelling and biological parameter estimation techniques have merit for refining our current knowledge to improve the management of this resource in future.