Influence of climate-driven low oxygen zones on fish biodiversity: A case study from the Arabian Sea

Fish biodiversity is at risk globally due to the climate-driven expansion of areas of low dissolved oxygen. The Arabian Sea is one of the three regions globally with a persistent oxygen minimum zone (OMZ). The Arabian Sea OMZ is a consequence of the high productivity associated with the Southwest Monsoon (SWMon). During the Northeast Monsoon (NEMon), the OMZ is also present but tends to be found in deeper water (>200 m). I examined patterns in fish biodiversity across the Arabian Sea with respect to region, season and depth strata and in relation to environmental conditions, including bottom sea temperature, salinity and dissolved oxygen. Analyses were based on 764 trawl samples collected as part of a stock assessment survey conducted between 2007 and 2008. A total of 99,319 fish were collected, representing 207 species. There was no variation in the estimated total species diversity by region, and the number of species was highest during the NEMon and decreased with greater depth. The average species richness per trawl also did not vary with region and was highest during the NEMon and lowest in the post-Southwest Monsoon (PostMon) period and was greatest in the shallowest depth strata. The Chao1 and Shannon indices of biodiversity showed no regional patterns but were highest in the NEMon and lowest in the SWMon. The analysis also showed declines in these two indices with increasing depth. The total abundance was significantly higher during the PreMon and was invariant in three depth strata 1, 2 and 4 and low in depth strata 3. Biodiversity indices were poorly explained by the environmental variables. Climate change is expected to strengthen the SWMon, which will expand the OMZ. As the SWMon period and its expanded OMZ are associated with lower species richness, biodiversity is likely to be negatively affected by climate change. The generality of this prediction should be evaluated by exploring patterns in biodiversity associated with other OMZs globally and as these features expand through time.