Blue grenadier

Abstract: The fishery can be divided into two subfisheries (‘spawning’ and ‘non-spawning’). Commercial catch rates for the ‘non-spawning’ subfishery declined from the late 1980s to 1997, whereas those for the ‘spawning’ subfishery exhibit no obvious temporal trend. An ‘Integrated Analysis’ assessment, of the feasibility of reconciling these differing trends, uses catch (landed and discarded), catch rate, length-at-age, and catch-at-age data and estimates of absolute abundance based on the egg-production method. It emphasizes uncertainty due to model assumptions and the data included in the assessment. Use of the discard data allows more precise estimation of the magnitude of recent recruitments. Spawning biomass is estimated to have declined from a peak in 1989–91 to 1999 although fishing mortality has consistently been <6%for each subfishery. One main reason for the reduction in population size is the weakness of year-classes spawned from 1988 to 1993. Differences in catch rates between the two subfisheries can therefore be explained by interactions between the components of the population harvested by the two ‘subfisheries’, and the trends in year-class strength. A risk analysis is used to evaluate the consequences of different future levels of harvest for different assessment assumptions. Overall, the spawning biomass is predicted to increase over the next five to ten years as a result of the strong 1994 and 1995 year-classes, although the extent of this increase remains uncertain.

Abstract: Accelerator mass-spectrometry was used to measure radiocarbon in the earliest formed portions of selected blue grenadier, Macruronus novaezelandiae, otoliths to provide a validation of fish-age estimates based on the quantification of opaque and translucent zones in otolith thin-sections. Δ14C data from blue grenadier otoliths were compared with previous estimates of Δ14C in seawater-dissolved inorganic carbon at similar latitutes, longitudes, and depths to link variation in otolith Δ14C to time. Minimum otolith Δ14C was −76.9 ± 7.7‰, indicative of pre-bomb radiocarbon levels below the surface mixed-layer at latitudes where juvenile blue grenadier are found. When plotted versus fish age estimated from otolith sections, the majority of the Δ14C data combined to define a curve reflecting the increase in bomb radiocarbon in temperate oceans of the Southern Hemisphere, indicating that age-estimation procedures based on otolith thin-sections are satisfactory for determining blue grenadier age. If otolith-section age estimates were correct, peak otolith Δ14C of 106.8 ± 7.9‰ occurred during the late 1960s, i.e. earlier than expected. This may be a manifestation of an increase in the mixed-layer depth associated with increased frequency of zonal westerly winds at this time.

Abstract: Spatial and temporal variation in allele frequencies at 10 polymorphic loci were investigated in blue grenadier from Australian waters. Little geographic differentiation was found among three major regions. Nearly all of the detectable variation (>99%) was found within samples, while variation between samples taken at the same locality accounted for most of the remaining variation (0.8%). Blue grenadier were polymorphic at 22% of the 46 loci initially screened (P0.99= 0.22). Overall mean heterozygosity was 0.068±0.018. This value is considerably higher than has previously been reported for this species. Almost 700 fish were aged and typed for genetic variation. Fourteen age-classes (0+: 2-14+ years old) were compared. Little genetic difference was observed among age-classes within regions, or in the overall sample. A significant difference was found between sexes at the Est-l locus; this was due to an increase in males homozygous for the Est-l104 allele in the eastern Tasmanian sample taken during August 1984. The same sample displayed a significant shift in allele frequency at the Sod locus. This sample was taken during the spawning season of blue grenadier on the west coast of Tasmania and may provide circumstantial evidence of differential spawning migration by fish with particular genotypes from eastern Tasmania to the west coast. Comparisons of samples from Australian waters with a sample of fish from New Zealand showed significant heterogeneity at 6 of the 11 loci polymorphic in the two areas. The observed differentiation indicates that blue grenadier from New Zealand are genetically isolated from those of Australia. However, the apparent genetic homogeneity observed among the Australian samples suggests that, in the absence of indications to the contrary, blue grenadier stocks throughout south-eastern Australia can be treated for management purposes as part of a single, interbreeding unit.

Abstract: In situ examination of the behaviour of fish was undertaken with underwater cameras positioned on demersal trawl gear used by Australia's South East Trawl Fishery. Blue grenadier (Macruronus novaezelandiae), pink ling (Genypterus blacodes) and whiptails (Coelorinchus spp.) swam in an anguilliform mode whereas other species displayed a carangiform swimming mode. Tiger flathead (Neoplatycephalus richardsoni) and ocean perch (Heliocolenus spp.) were active in response to the approaching trawl net compared with the generally passive activity of whiptails, New Zealand dory (Cyttus novaezelandiae), and jackass morwong (Nemadactylus macropterus). However, when in the body of the trawl, gemfish were active while ocean perch, whiptails and New Zealand dory were generally passive. Some blue grenadier, ocean perch and whiptails escaped capture by passing through open meshes in the trawl mouth, whereas tiger flathead passed under the ground gear. In the trawl body, small numbers of blue grenadier passed through open meshes in the top panel whereas numerous spotted warehou swam faster than the towing speed, presumably escaping capture by swimming forwards and out of the trawl. Interspecific behavioural variation in escape response could be utilised to design more efficient trawl gears.