A modelling approach to identify habitat suitability for Indian squid (Uroteuthis (Photololigo) duvaucelii) in the northeastern Arabian Sea

The ocean is not a static blue expanse; it is a living, breathing system where temperature, currents, and light shift with the seasons, dictating where life can thrive. For marine biologists, understanding these shifts is a matter of survival for the species they study and the people who rely on them. Many sea creatures, particularly those with short lives and fast growth rates like squid, are exquisitely sensitive to their surroundings. They do not wander aimlessly; they track the specific conditions that allow them to eat, grow, and reproduce. When the environment changes, these animals move, and the fishing fleets that depend on them must follow. If scientists can predict where the water will be right for a species, they can help manage the fishery so that it remains productive without depleting the stock. This is the core challenge of habitat suitability modeling: using data to map the invisible preferences of a species across a vast and changing ocean.

In the northeastern Arabian Sea, off the coast of India's Maharashtra state, a team of researchers set out to solve this puzzle for the Indian squid, a commercially vital species. They wanted to know exactly what combination of ocean conditions creates the perfect home for these squids. The team gathered data from 280 trawl hauls conducted between 2017 and 2019, recording where and when fishermen caught the squid, how much they caught, and the depth at which they fished. They then overlaid this fishing data with satellite information about the ocean's surface. They looked at sea surface temperature, the height of the sea surface, salinity, the amount of light available for plant growth, the speed of ocean currents, and the concentration of microscopic plant life known as chlorophyll. By feeding all these variables into a statistical model, the researchers could identify which specific environmental factors were driving the squid to certain locations.

The study revealed that the squid do not stay in one place. Their distribution follows a distinct seasonal rhythm tied to the monsoon. After the fishing season resumes in August, following a mandatory ban during the heavy rains, the squid are found in a concentrated band between 17 degrees and 16 degrees north latitude, in waters ranging from 20 to 40 meters deep. By September, the fishing fleet and the squid both move southward to the 16 to 15 degree latitude range, where the water is slightly deeper, between 40 and 60 meters. As the season progresses, the squid become more scattered and spread northward again, reaching as far as 20 degrees north. This movement suggests the animals are tracking a shifting zone of productivity created by the changing monsoon winds and currents.

The model identified a very specific set of conditions that signal a high probability of finding Indian squid. The squid prefer waters where the sea surface height anomaly is between 0.2 and 0.4 meters, a range that often indicates nutrient-rich areas where eddies and currents concentrate food. They thrive in salinity levels between 34.5 and 35 parts per thousand and in water temperatures between 27.5 and 29.5 degrees Celsius. Crucially, the squid are found where there is strong light, with photosynthetically active radiation exceeding 40 units, and where the water contains more than 2 units of chlorophyll, indicating a rich food web. They also favor areas where the current speed is faster than 0.3 meters per second. When all these factors align, the model predicts a high abundance of squid, particularly in a zone between 15 and 16 degrees north latitude and 71.5 to 72 degrees east longitude.

The researchers found that the season and the specific location were the most powerful predictors of where the squid would be. The model successfully recreated the known areas of high catch, confirming that the squid are not randomly distributed but are tightly linked to these environmental cues. The study suggests that the squid are responding to the recovery of coastal productivity after the monsoon, moving to areas where mixing and circulation have created ideal feeding grounds. This behavior highlights how closely the fishery is tied to the natural rhythm of the ocean. By mapping these preferences, the study provides a tool for fisheries managers to understand the distribution of the resource on a broad scale. Regular updates to these habitat maps could help ensure that the Indian squid fishery remains sustainable, allowing the industry to adapt to the shifting ocean conditions that govern the life of this fast-moving, short-lived species.