An assessment of the spatial extent and relative importance of nurseries, and of the genetic structure among nurseries of rig (Mustelus lenticulatus), an endemic New Zealand shark

Abstract

The spatial extent of nursery grounds for rig (Mustelus lenticulatus) was examined using data from a national gill netting survey, and from the Ministry of Fisheries trawl survey database. Primary nurseries exclusively occur within harbours, inlets and estuaries for the North Island, but there appeared to be a northern limit at approximately 35° S, despite seemingly suitable habitat. Primary habitat occurs within inlets and harbours in the South Island, but may also be situated in ocean beach and/or semi-protected environments where typical nursery habitat is sparse or lacking. Secondary nurseries are geographically associated with the primary nurseries but cover a much wider area, and are typically located in shallow coastal or semi-protected embayments. The relative importance among rig nurseries was investigated by CPUE analysis of the gill net and trawl survey data. Kaipara Harbour, and the north-west coast of the North Island (NWCNI) appear to be the most important areas of the North Island in terms of juvenile abundance, which may relate to the area of available habitat and the distance of most nurseries from urban/industrial development. Golden Bay and the west coast of the South Island have similar importance, and recruitment of rig in these areas appears to be linked, despite the CPUE of the adult stocks following different trends. Temporal patterns in 0+ CPUE were also investigated in Pauatahanui Inlet and Porirua Harbour near Wellington. The results suggested that juvenile abundance has declined in these nurseries since the 1980s, but has recently stabilised. Suggestions were made to explain these declines in terms of habitat alteration by anthropogenic activities. Growth and size structure in 0+ rig populations were also examined on both spatial and temporal scales, and the mechanisms responsible for the patterns found have been explored. These analysis also revealed a significant correlation between the birth size of rig and the sea surface temperature during development. The development of a predictive index, which may be useful for predicting future year class strength, was discussed. The genetic population structure of rig was investigated using mitochondrial DNA RFLP and allozyme analysis. Neonate rig were sampled from North Island nurseries, and additional adult samples were obtained from Tasman Bay. Very low levels of variation were detected by both methods, which hindered the analysis. A distinct haplotype was identified from two nurseries on the NWCNI, although results of an analysis of geographic heterogeneity in mtDNA haplotype frequencies using a Monte Carlo simulation were non-significant. If genetic structure does exist among regions, it is suggested that detection would rely on higher resolution genetic techniques (PCR/sequencing of D-loop and/or micro-satellites). A main objective of the study as a whole was to identify areas of special importance for the species, in terms of nurseries that are supporting large juvenile populations, and of juvenile populations within these nurseries that may be genetically distinct. The north-west coast of the North Island has been identified as being of special importance for the former criterion, and there are preliminary indications that the same region may also be of importance for the latter.