In the freezing waters of Patagonia, southern Chile, the black southern codfish takes what it can get. With different levels of salinity and nutrients at the mouths of fjords and channels, the black southern codfish maintains regional feeding habits, a new study has found.
Researchers Matthias Hüne and Rodrigo Vega, from the Institute of Ecology and Biodiversity (IEB) and Fisheries Development Institute in Chile, collected fish samples in gill nets to observe and evaluate how the feeding pattern variation of black southern cod is influenced by oceanic and continental water in the Staples Strait in Captain Arancena Island and Puerto Bories in the Ultima Esperanza fjord. Since the black southern cod, an ice fish, is extremely abundant in Patagonia, Hüne and Vega wanted to better understand the trophic structure of the species, which will contribute a more complex understanding of the trophic ecology of fish in Chile.
In the coming years, glacier melt is likely to reduce the salinity in surrounding oceans in these regions off southern South America. As a result, the diversity of prey species for the black southern cod will most likely be promoted, meaning that the oceanic food webs may become more complex. It is plausible that those fish will not be intensively affected by changing climate because they are highly adaptable to both higher temperature and lower salinity environments.
By dissecting fish samples, the authors of the study were able to determine the diet composition of the species in selected regions. The authors investigated the spatial differentiation in diet composition of black southern cod by taking into consideration both environmental factors (salinity, temperature, oxygen concentration) and biological factors (gender, size). Through this study, they were able to develop a series of factors that predicts the spatial variation for the diet of the fish.
Ultimately, Hüne and Vega ascertained that in Staples Strait, the black southern codfish primarily preys on polychaetes, multi-segmented worms that have fleshy bristles protruding from each segment and which are present from abyssal depth to rocky shores.In Puerto Bories, however, the black southern codfish preys mainly on algae and on crustaceans, including ostracods (“seed shrimp” with two shells that exist in almost all aquatic environments, including hot springs) and gammarids (shrimplike creatures which can swim upside-down, backwards or on their sides). Even though there is hardly any difference in the diet pattern among different sexes of the fish, small-sized black southern cod were found to have relatively lower proportion of empty stomachs.
There is no doubt that environmental variables play a vital role in affecting the feeding variability of the black southern cod. Theoretically, the fish is prone to consume more diverse prey items where there is higher temperature and lower salinity, which accounts for the relatively high prey diversity in Puerto Bories. The adequacy of potential prey species could largely be attributed to suitable environmental conditions. According to Clarke and Johnston, the metabolic rate of fish in warm water is anticipated to be comparably higher, which could be an explanation for the phenomenon.
Furthermore, the research found that small-sized black southern cod mainly prey near the surface of the ocean (or in benthic zone), where the majority of their food comes from ostracods and gammarids. Nevertheless, large-sized species spend most of their time in preying on polychaetes in the water column. Hence, there is no surprise that most of the black southern cod from Puerto Bories were smaller than specimens from Staples Strait.
In conclusion, the black southern cod from different locations exhibits various feeding patterns, in which temperature and salinity of the ocean, as well as their physical condition, play an important role in shaping diet. Generally speaking, diversity of prey items is positively correlated with high temperature and low salinity. However, quantity or size of prey is positively associated with high salinity. In other words, the black southern cod from ocean-influenced regions tend to prey on more diverse food sources; and fish from continent-water-influenced regions feed on larger species and have heavier stomach contents. As a result of these qualities, this species seems resilient in the face of climate change.