INVESTIGATING LENGTH AT MATURITY OF ANTARCTIC TOOTHFISH (DISSOSTICHUS MAWSONI) BASED ON SCIENTIFIC OBSERVERS’ DATA

Through generalised linear models, the gonadosomatic index of Antarctic toothfish in the Ross Sea region was shown to vary with latitude, length and month. Limitations of the scientific observers’ staging data were highlighted and GSI is recommended as a better indicator of maturity status in Antarctic toothfish. Reports of histological analyses of a small number of Antarctic toothfish showed vitellogenic fish with low GSI values, in some cases below 1%. In most studies there was little if any difference in the GSI values of fish at different maturity stages. However these studies were carried out on a limited number of fish, most of which were from the southern area, therefore not on obviously spawning fish. Histological analysis using a hindcasting assessment of 683 samples collected in December to February showed most fish on the shelf had not spawned that year, most fish in the north had spawned and the shelf contained a mixture of fish that had spawned or not, with length at 50% maturity of about 137cm. The equivalent GSI at 50% maturity was in the range of 1.1 to 1.4%. These results were used to estimate GSI thresholds for fish that had spawned in the previous season; which were set at 1% and 1.5% GSI, but are limited to only female fish in the sampled areas for December through February. Based on both histology and GSI data, most fish found in the northern areas, about a third of those found in the slope areas and very few of those found in the southern areas had spawned. As the GSI values of fish caught in the northern area were never very low, it is expected that all the fish in the northern areas spawn every year when in that area. If they were resting in the north it is expected residual GSI would be lower. Conversely, as the GSI values of fish caught in the south were very low, fish caught there are not expected to spawn in the current year, nor are they expected to have migrated back from the north; or their residual GSI would be higher, in the order of 1% or more. Therefore any movement would have to be between the north and the slope areas, with only a proportion of fish coming back to the slope since only a third there are mature. Any other movements would have to be outside of the fishing season, for example a yearly northern migration during spawning season only. Lengths at 50% maturity of Antarctic toothfish were calculated for each SSRU or area in the Ross Sea region; they varied from about from 89 to 150cm for females and 36 to 184 cm for males, from north to south respectively. Length at 50% maturity was also calculated for female fish from the slope, which was similar to the value calculated from GSI of 137cm. Uncertainty in the oocyte development cycle may create biases in different histological assessment methods which may influence estimates of length at maturity or GSI thresholds. Length distributions are also known to be spatially heterogeneous. A population-wide length at 50% maturity can therefore not be determined without the help of a spatially-explicit population model and any length at maturity value should be treated with caution. Gonadosomatic index has shown promise as a potential index of Antarctic toothfish maturity. However, further work is recommended in order to improve the current knowledge of toothfish maturity...continued