Low Oxygen Zones Predict Future Condition of Fish Under Climate Change

Oxygen concentrations are predicted to decline under climate change scenarios. To assess the possible effect of low dissolved oxygen levels on fish condition, we evaluated the condition of fish in the Northwest Arabian Sea, a region of persistent oxygen minimum zones (OMZs). Condition of fish was inferred from the coefficients of length–weight relationships (LWR), comparing LWR coefficients for 53 species sampled across the Northwest Arabian Sea OMZ to the coefficients reported for these species from non-OMZ regions. Regional effects of oxygen depletion were also examined by comparing coefficients from LWR of seven fish species in four different regions of the Northwest Arabian Sea across a latitudinal gradient. The estimated values of a, the body form coefficient, were significantly higher in the Northwest Arabian Sea than in non-OMZ regions. However, there was no significant difference in b, the allometric growth rate, observed in the Northwest Arabian Sea with those observed elsewhere. Regions showed significant difference in allometric growth rates for five of seven investigated fish species, with Drepane longimana, Pagellus affinis, and Pomadasys commersonnii showing decreasing trends from north to south, while Argyrops spinifer and Carangoides equula showed the opposite trend, and Cheimerius nufar and Plectorhinchus schotaf showed no discernable trend. Fishes from the Northwest Arabian Sea had larger body forms (as indicated by the LWR coefficient a) compared to conspecifics in non-OMZ regions but showed increased allometric growth rates (as indicated by the LWR coefficient b) with declining oxygen on a regional basis. Climate models predict expansion of OMZs globally, and fishes in the Arabian Sea showed unexpected responses in relation to the OMZ. Consequently, the conditions of the fishes need to be studied in Arabian Sea using empty weight during different seasons, regions, and depth strata and configure its relation to the environmental factors and compare the results with same fishes from non-OMZ.