Is there a significant long-term shift in phytoplankton in small pelagic fish diets along India's southwest coast?

The two main small pelagic fishes along the southwest coast of India in the eastern Arabian Sea, the Indian oil sardine (Sardinella longiceps) and the Indian mackerel (Rastrelliger kanagurta) were studied to see if there was any notable long-term alteration in phytoplankton in their diets. The basic oceanographic features and the long-term sea surface warming in the study region are first examined using satellite data, which reveals clear seasonality in the wind, sea surface temperature, sea surface salinity, and chlorophyll-a as well as long-term ocean surface warming. We then considered a recent gut content data set of Indian oil sardine and Indian mackerel from the 2010–2011 period from two landing places along India's southwest coast, which was compared with numerous historical data sets to find out if there was any long-term phytoplankton compositional change in the diet. The recent data revealed the same dominant phytoplankton as those collected decades ago, such as Coscinodiscus, Nitzschia, Pleurosigma, and Thalassiosira in sardines and Coscinodiscus, Thalassiosira, Ceratium, Dinophysis, Protoperidinium, and Pyrophacus in mackerel. This suggests the lack of any significant long-term qualitative shift of phytoplankton in the Indian oil sardine and Indian mackerel over the last several decades. However, several phytoplankton genera found present in the recent data were absent in the historical data, including 10 genera for mackerel and 17 for Indian oil sardine. These recent phytoplankton records are intriguing, and we feel they could be an early sign of long-term phytoplankton compositional or relative abundance shift, although additional research is needed to confirm this.