Ecology of the Antarctic Sea Ice Zone Symposium Session: Benthic communities and diversity Distribution and abundance of demersal fish and invertebrate fauna around the South Orkneys Islands Instituto Español de Oceanografía SPAIN Ignacio Olaso, Ana Ramos, Eduardo Balguerías & Javier Pereiro
FISHERY UNTIL 1986 SOUTH ORKNEY ISLANDS
Study area BOTTOM TRAWL SURVEY IN SUMMER 1991 Location of 130 hauls ( ) made during the cruise ANT9101 Narrow SOUTH ORKNEY ISLANDS Random sampling stratified Three strata depth Each haul= 30 minutes 150 m 250 m Mud 500 m
Aims: To relate the diets of demersal fish species with the invertebrate fauna To estimate abundances of the main demersal and benthic species To elaborate fish diet overlap models T i k l d f th t hi t t f th To improve knowledge of the trophic structure of the demersal system
Methods. Data analysis All fish species separated, length inferior cm, weighted and counted by haul. Megabenthic invertebrates separated into 24 groups, weighed and counted by haul Densities were calculated on the sweep area method, 0.021 mn 2.
Methods. Data analysis 2500 stomach contents of ten most important fish species (more than 90% of total number and biomass) were analyzed on board volumetrically (cc) Maximum 10 stomachs by lengt range by fish species and haul Food. quantity (the entire stomach contents) and food quality (food composition) were determined. Prey items were identified to the lowest possible taxon. The relative importance of individual prey taxa was assessed using percentage volume (%V)
Results Abundance in weight (Kilos) of Champsocephalus gunnari and Gobionotothen gibberifrons Total fish 33 % N G. 19.4 % W 25 % N 28.3 % W
Biomass distribution of Fish. Cruise ANT9101 The highest densities of demersal fishes are found to the g northwest of the island, and diminish progressively towards the southeast as species show less dependence on areas.
Biomass distribution of Fish. Cruise ANT9101 G. gibberifrons Wide distribution 124 trawls
Biomass distribution of Fish. Cruise ANT9101 C. gunnari Concentrated distribution
Biomass distribution of Fish. Cruise ANT9101 C. rastrospinosus Concentrated distribution
Food diet of the ten most important fish species. Data expressed in percentage by volume Krill feeders Fish Krill feeders feeders Krill feeders Krill & Fish f. Fish feeders Benthic feeders
Results Diet overlap of the ten most important fish species III I I I I III I I I I Specialist feeders (Krill) III I I Fish hfeeders II Benthic feeders
Results Food diet of the ten most important t fish species, represented in groups of fish; data expressed in %V
Results Food diet of the ten most important t fish species, represented in groups of fish; data expressed in %V Amphip.
Results Diet overlap of the 22 fish groups, pertaining to the ten most important fish species in south Orkney Islands D C B D Fish feeders C Krill and benthic prey B Generalists A Krill specialists A
Results Frequency of occurrence of different benthic groups in the cruise ANT9101 Invertebrates were collected at 95.4% of the stations. Frequency % The most widely distributed were Echinodermata and Cnidaria (Asteroidea, ) Frequency %
Results Relative abundances in number (%) of the different benthic groups collected during cruise ANT9101 ABUNDANCE % Number The greatest abundance in number corresponding to Holothuroidea
Results Relative abundances in weight (%) of the different benthic groups collected during cruise ANT9101
Distribution of chlorophyll a concentration High High High Poor High Poor Poor
Patchy distribution, typical of Antarctic epibenthic communities Biomass distribution of invertebrates. Cruise ANT9101 High Hard bottoms Filter feeders Lowest densities Deep areas Lowest densities South muddy shelf Kg/haul
Biomass distribution of invertebrates. Cruise ANT9101 KRILL Shell break around 300 m ACTINIAS Greatest densities from 150 m SPONGES ASCIDIAS Between 150 300 m Abundance in the internal shelf area HOLOTHURIAS & OPHIURAS > 150 m Kg/haul
Biomass distribution of invertebrates. Cruise ANT9101 KRILL Shell break around 300 m Kg/haul
This paper describes the feeding habits of the main demersal fishes, and describes diet overlap models and relates these models to predator length and prey abundance. In the first of them, the classical fish predators are found, with large mouth sizes. In the second their habits are more benthic, and with the exception of species that grow to great lengths, Notothenia neglecta, they are opportunistic species preying on benthic fauna and krill. Building a trophic scheme of the most important demersal fish species allows us to link the energetic consumption of benthic invertebrates by fish. Position of Fishes in the Food Web Formal modelling of the inter-specific relationships among species in predator-prey models that clearly indicate the direction of the energetic flow to demersal fish.
Graphic representation of fish species and benthos mobile preys in the south Orkney Islands Formal modelling of the inter-specific relationships among species in predator-prey models
Major conclusions The demersal ictiofauna of the Orcadas is very similar to that of the Elefante, it seems characteristic and stable, dominated by species of the families Notothenidae y Chaennichthyidae Over 80% of the fish species have a Low Antarctic distribution Approximately 20% (10738 kilos) correspond Fish feeding to on the benthic benthic invertebrates, fauna the rates are low, with which sexual maturity is only reached at 7 or 8 years. The rate is greater for planctophagous fish or krill feeders (C. gunnari o N. rossii).
Study area Location of 120 hauls ( ) carry out during the cruise ANT9101 SOUTH ORKNEY ISLANDS
Study area Location of 120 hauls ( ) carry out during the cruise ANT9101