Review of the life history and fishery characteristics of New Zealand rig and school shark. New Zealand Fisheries Report 2010/2, January 2010. 

Blackwell, R G., and Francis, M. P. 

2010 

Ministry of Fisheries 

 

Report 

The Quota Management Areas (QMAs) for rig (Mustelus lenticulatus) and school shark (Galeorhinus galeus) were established in October 1986, when limited information was available on the stock relationships of these species. The five rig stocks around mainland New Zealand (excluding FMA 10) were established at a spatial scale consistent with tagging information, while the seven school shark stocks used Fisheries Management Area boundaries as a conservative measure. Elasmobranchs like rig and school shark have relatively slow growth rates, low reproductive capacity, and low productivity, and may be susceptible to over-fishing and local depletion. A mismatch between biological stocks and their management areas could result in sub-optimal management and overfishing. In this report we review biological and fishery data that have become available since 1986 to determine whether they provide any indication of such a mismatch. The sources of information available for testing the validity of rig and school shark Fishstock boundaries are very limited. No significant differences in rig growth rate were found among SPO 1 East, 3, and 7 Fishstocks. Length and age at maturity were indistinguishable between SPO 3 and 7, but rig in SPO 1 East apparently matured at a smaller size than did South Island rig. There is no indication of a mismatch between the location of rig nursery grounds and the boundaries of rig Fishstocks – all Fishstocks contain at least one nursery ground. Length-weight regressions do not differ between SPO 3 and 7, and vertebral counts are similar throughout mainland New Zealand. Higher vertebral counts in SPO 10 are attributable to the existence there of a different undescribed species of Mustelus. For school shark, no information is available for comparing morphology, growth rates, length and age at maturity, or weight-at-length among Fishstocks. School shark nursery areas appear to be restricted to mainland coastal waters between the Hauraki Gulf and Kaipara Harbour in the north and Oamaru and Jackson Bay in the south. Nursery areas are not known for SCH 4 and 5, or from Northland in SCH 1, despite considerable trawl survey effort in all three regions. This suggests that school shark populations in SCH 4 and 5 are maintained by recruits from central New Zealand (SCH 3 and 7 are the nearest Fishstocks that have nursery grounds). Catch composition data and sex ratios obtained from commercial fisheries and trawl surveys provide little information on stock identity. For rig, there were no clear differences in size composition or sex ratio data that might indicate the existence of different stocks. For school shark, there may be a real difference in size composition between SCH 5 and elsewhere. However, this is not necessarily indicative of a distinct stock – an alternative explanation is that larger sharks migrate southwards from mainland New Zealand to the Stewart–Snares Islands shelf. For rig, clear long-term differences were observed in two sets of catch per unit effort (CPUE) series from adjacent areas. The Manukau Harbour (SPO 1 West) index showed a long-term decline followed by stabilisation and slight recovery, whereas other adjacent SPO 1 subareas and SPO 8 showed relatively stable indices over the same time period. Similarly, contrasting trends were observed for west coast South Island (increasing) and Tasman and Golden bays (decreasing) in SPO 7. These patterns suggest that movement of rig may be insufficient to homogenise differences in population density and trajectory over relatively small spatial scales, but stock separation at the indicated spatial scale seems unlikely. The CPUE differences may result from processes acting below the stock level. 

Biodiversity, Fisheries 

School shark, rig, fisheries, life history, biology, Kaipara Harbour, fishstock, catch composition, quota management area, size composition 

10_02_FARBlackwellFrancis2010_Review_lifehistory_and_fishery_charact_rig_sch_shark.pdf