Icefish age groups and growth.

Until recent decades, Antarctic resources were regarded to belong to no one and, therefore, were harvested without restrictions. This exploitation ultimately led to the near-complete exhaustion of economically valuable Antarctic animals. The process began with the largest whale species, followed by seals, smaller whale species, porpoises, penguins, large species of icefish and now krill. At present further exploitation is planned and administered sustainable fishery which needs to be based on estimations of all age groups of fish. However, that is difficult to the obtain, because of inaccessibility of the icefish’s habitat of sharp rocky peaks and the larger shelf depths. Unique icefish biology determine age estimation only from otoliths (lack of scales and bone reduction). However interpretation of annual otolith increments performed by specialists from around the world provided very different estimates of the age groups in the range of 1 to 17 years with an error of 8 ys for the same otolith. Manual accounting of 2000 daily increments in 2 mm otoliths radius is terrified too. Therefore, I performed detailed studies of Ps. georgianus, Chaen. aceratus and Champ. gunnari otolith microstructures to determine how and why they form and to relate particular patterns and composition to growth, age groups, and life histories. Icefish ages were automatically estimated from daily increments in otoliths that were verified with age estimation for large ichthyologic samples by the modal frequency of otolith mass time series to fit superimposed normal distributions of age group of juveniles and adults. They were verified also by age estimation from body lengths. The long time series data of icefish that were hatch every year in July-August-September and caught every year in December-January – in the same time and place since 1970 show separate age group at length by the length frequency analysis. Each age group constitute close neighbors in body length (TL) that have the same age and are larger than previous length group by one year increment of 6-10 cm of body until the smaller one was only preceded by 1.5 cm hatched larvae. We determined that maximum ages were 8 y for C. gunnari, 7 y for C. aceratus, and 6 y for P. georgianus. Sizes at first maturity were became smaller for icefish males than females in order to adaptation to vertical distributions. In upper waters C. gunnari females matured at about 32 cm and 5 y; in middle pelagic P. georgianus females matured at about 46.4 cm and 3.3 y; and in bottom adults C. aceratus females matured at about 61.5 cm and 5.6 y. C. aceratus females were 2.5 cm longer than males in age group 5 and in age groups 6 and 7+ we found only females. The females of the other two species averaged 2 cm longer than males among which sex size differences were smaller for C. gunnari. Older, larger fish can be hidden in the hilly rocky bottom and avoid bottom nets. The otolith masses and shapes of the three icefish species varied with their body morphologies and levels of bone and muscle reduction, that is the largest for bottom adults C. aceratus and lack for upper small C. gunnari. The relationship between body mass and total length varied by species, habitat occupied, thermal regime, diet, individual life histories, season, and growth rate. Icefish have responded to exploitation and climate change by different feeding strategies. The smaller, slender pelagic species, C. gunnari, has shifted their organizational level of feeding from individually to previously not know large pelagic swarms having larger encounter probabilities of pelagic schools of small krill and appropriate to warming decreased substantially in size at maturity. The largest benthic species, C. aceratus, has remained relatively large by remaining as adults in colder, deeper waters that have shifted from feeding on krill to feeding on large benthic animals. The size at maturity of the mid-sized, stocky semipelagic species, P. georgianus, has been reduced by reductions in ice-algae feeding Euphausia superba as a result of warming. Larger P. georgianus were found congregated at whirls that concentrated larger older krill driven from south cold Weddell Sea on east shelves where inshore is the spawning ground too and the smaller icefish migrated after hatch with along shore current to western shores having inflow from south of the fresh small krill after hatch in warmer South Shetland waters - that both different distributions further rely on changes in pressure systems and ice cover. Recently, abnormal otoliths have been observed, reducing their mass and shape (loss of edges, replacement of aragonite with lighter vaterite). Those deficiencies and defects correspond to the effects and nature of dioxin activity multiplied by their uptake and transport as oxygen through skin in icefish. Monitoring of dioxin concentration is difficult and costly, requires protection against the toxic effects. Thus, cheaper, safer methods of analyzing the natural signs of otoliths resulting from the influence of dioxins on icefish biology and existence are needed to reduce the risk of their destruction.

Presentation Preference: Oral

Primary Presenter: Ryszard Traczyk, AFS (ryszardtraczyk@gmail.com)

Authors:

Ryszard Traczyk, ASLO, AFS  (ryszardtraczyk@gmail.com)