VIWA Taihoro Nukurangi Movement patterns and stock mixing of blue cod in Southland (BC05) G. Carbine, J. McKenzie Final Research Report for Ministry of Fisheries Research Project BCO9702 (Revised) National Institute of Water and Atmospheric Research February 2001 Final Research Report Report Title: Movement patterns and stock mixing of blue cod in Southland (BC05) Authors: Glen Carbines and Jeremy McKenzie Date: 30 September 2000 Contractor: National Institute of Water and Atmospheric Research Limited 3. Project Title: Determination of movement of blue cod in Southland 4. Project Code: BCO9702 5. Project Leader: Glen Carbines 6. Duration of Project: Start Date: 1 October 1997 Completion Date: 30 September 1999 7. Executive Summary: Movement patterns of blue cod are currently unclear, and it has not been established if populations can be regarded as a single entity within some of the present fish stock areas. The current study sought to determine movements of blue cod in domestic return area 025 within the Southland fish stock BC05. During March 1998, 9368 blue cod were tagged within domestic fishing return area 025 (Foveaux Strait). The tagging programme used a balanced stratified series of replicated sites (n=9) within three latitudinal and three longitudinal strata spread throughout area 025 (approximately 1000 fish per stratum). Three different bottom types were also identified throughout most sites (sand flats, biogenic reefs, and rocky reef fringe). Blue cod were tagged using t-bar tags in a ventral position. Tagged fish ranged in size from 140-510 mm total length, with a mean of 324.5 + 0.5 mm. Length frequency distributions were derived for all sites and bottom types. The larger numbers of small fish caught on sand flats and biogenic reefs suggest these habitats may be important to recruitment. CPUE results also indicated that blue cod abundance might differ between bottom habitats. After 20 months, 743 (7.9%) blue cod had been returned from all sites and all bottom types present within those sites. The largest distance moved was 156 km, however the median was 790 m, with 60.2% of fish moving less than 1 km. It therefore appears that densities of blue cod in Foveaux Strait are temporally stable on a median-term scale. No size distance relationship was evident in tagging data, and habitats blue cod were released into also had no effect on the distance moved. Spatial location was an important determinant of distance travelled. However, a significant interaction between latitudinal and longitudinal release strata shows that factors influencing distance travelled are more complex than the latitudinal or longitudinal location on its own. Mixing rate calculations showed that blue cod in area 025 had moderate mixing with neighbouring domestic return areas (up to 14.7%). However, mixing was considerably higher between adjacent sub-areas within area 025 (up to 44.1%). These results suggest that blue cod stocks are relatively stable at the domestic return area scale. While the majority of blue cod did not move any significant distance, those that did showed a strong trend toward counter current north-west movements. Mixing rate calculations indicated higher levels of immigration to the west and emigration to the east of area 025. An increased proportion of blue cod moving significant distances in the spring of both 1998 and 1999 is suggestive of a possible return seasonal migration associated with spawning. Overall, these observations suggest that blue cod movement is complex. 8. Objectives: 1. To design a tagging programme to determine movements of blue cod in domestic fishing return area 025 in Southland. 2. To complete the tagging phase of the programme. 3. To complete the recovery phase of the programme. Introduction Blue cod (Parapercis colias) are endemic to New Zealand. They are not a true cod but a member of the weaver family (Pinguipedidae), of which 43 species are described throughout the Indo-Pacific region. Distributed from the shore to the shelf edge of New Zealand's entire coastline, blue cod are most abundant in Southland and the Chatham Islands. They are opportunistic benthic carnivores found on reef edges, shingle/gravel or sandy bottoms, often close to rocky outcrops (Graham 1939a, Rapson 1956, Mutch 1983). Blue cod occur primarily in coastal waters shallower than 200 m, however, they may occasionally be caught by trawlers from as deep as 350 m (Anderson 1998). Like most of New Zealand's reef fish, blue cod are not evenly distributed throughout their range of habitats, and several biological and physical factors appear to influence their distribution (Mutch 1983, Jones 1988). The response to these factors may also vary with age and may cause younger blue cod to have a more transient existence (Mace & Johnston 1983, Mutch 1983). Larval blue cod are thought to settle into offshore habitats deeper than 120 m and then move inshore upon reaching lengths of 180-200 mm (Rapson 1956). However, scientific divers have observed juvenile blue cod shallower than 20 m in both the Marlborough Sounds (Cole, 2000 Pers. Com.) and Paterson Inlet, Stewart Island (Pers Obbs. 1998). As adults, blue cod have been shown to remain within small-localised areas (Rapson 1956, Mace & Johnston 1983, Mutch 1983). There are also anecdotal observations that suggest blue cod off Dunedin (Figure 1) migrate off shore in late winter to spawn (Graham 1939b, Robertson 1973). However, there is no evidence of seasonal migration from studies of blue cod in the Marlborough Sounds (Figure 1) (Rapson 1956, Mace & Johnston 1983). Members of the family Pinguipedidae are usually territorial (Stroud 1982, Sano 1990, Clark etal. 1991, Kobayashi et al. 1993a, Kobayashi et al. 1993b, Sano 1993, Ohnishi et al. 1997), and large male blue cod are have been observed to hold large and rather loose territories (Mutch 1983). Underwater observations of blue cod show that male body size and territory size are positively correlated. Social groups of 3-5 females may exist within the home range a single dominant male (Mutch 1983). Despite being predominately territorial, blue cod have been observed to occasionally school. Many fishers have reported catches of single sexed groups of blue cod (Rapson 1956, Warren et al. 1997), and mid water aggregations have been observed by scientific divers (Duffy, 1995 pers. comm.; Francis, 1998 pers. comm.). Analysis of gut contents suggests mixed sex schooling may be a feeding response (Duffy, 1995 pers. comm.) Blue cod are an important commercial and recreational fishery in the South Island of New Zealand (Annala 1999). The commercial blue cod catch is managed under the Quota Management System (QMS) relative to eight Quota Management Areas (QMAs). Each QMA has been assigned its own annual Total Allowable Commercial Catch (TACC). The QMA with the largest TACC is BC05 (Figure 1). At 1536 tonnes, BC05 accounts for 58% of the National TACC. The BC05 TACC is estimated to be eleven times larger than Southland's recreational take (139 tonnes) (Annala 1999). Both recreational and commercial blue cod fisheries in Southland are constrained by a minimal legal size of 330 mm total length (Annala 1999). Within each QMA, there are several domestic fishing return areas (e.g. 7 for BC05, Figure 1). Fishers can operate in any domestic fishing return area within their QMA, and this may potentially lead to an uneven distribution of fishing effort. For example in BC05 all significant ports are located within domestic return area 025, consequently a high proportion of the annual BC05 catch (52% on average) is taken from this area (Warren et al. 1997). For the purposes of management, it is assumed that fish in each QMA can be considered a single "stock". In reality there may be several smaller discrete stocks within each QMA, and uneven distribution of fishing effort across the QMA may result in some of these stocks becoming over-fished. Additional management measures may be necessary to ensure fishing effort is proportional to abundance within sub-areas of QMA. In order to determine the appropriate spatial scale on which to manage blue cod within a QMA, it is necessary to gain information on mixing rates between areas. The first major blue cod tagging was undertaken in the Marlborough Sounds (Figure 1) between 1940-41 (Rapson 1956). Of the 5050 fish tagged, 194 (3.8%) were recaptured within 13 months; 83 within 3 months, 62 between 3 and 6 months, and 49 after 6 months. Eighteen (9.3%) of the recaptured fish had moved more than 1.6 km, and only six (3.1%) of these had moved more than 16 km. Of the 18 migrants, 16 were larger than 300 mm when tagged, suggesting that blue cod movement is size dependent. The largest movement recorded was 48 km, but as this was at the full range of fishing effort the possibility that tagged fish moved further could not be excluded (Rapson 1956). Rapson (1956) described these "migrants" as "solitary fish which for some reason have left the shoal and roved far afield". While Rapson (1956) also believed that occasional shoals of blue cod did migrate (based on a total absence of returns from some areas), in most localities there was evidence that shoals remained stationary for many months. Rapson (1956) concluded that blue cod from temporally stable groups: "... a few cod migrate, some small fish as rovers and the larger, when conditions are right, as travelling cod". A blue cod tagging programme was also undertaken in the Marlborough Sounds between 1973-76 (Mace & Johnston 1983). Anchor tags were used to mark 2430 blue cod. From these, 84 (3.5%) fish were recaptured; 43 within 3 months, 12 between 3 and 6 months, 9 between 6 and 9 months and 10 after 9 months.