Coastal Aquaculture and Mariculture

Unit 8- Deep sea fisheries

8.3.3. Locating Potential Fishing Zone (PFZ)
Oceanographic data that could be obtained from satellite includes: Sea surface temperature, ocean formations such as eddies, plums and internal waves, salinity, sea colour and red tides. These data are used to solve the effects of sea conditions on biological production. Studies on the characteristics of the marine (environment are important in fisheries research as the conditions in the sea play a major role in the availability offish. The fluctuations in the physical, chemical, and biological oceanographic conditions have a profound influence on the periodic and seasonal migration of fishes in the sea. The major parameters, which have bearing on the marine living resources, are temperature, salinity, upwelling, primary and secondary production.
i) Temperature
It is one way we measure ocean variability, but it is also an indicator of more complex ocean processes. Changes in surface seawater temperature affect the abundance and diversity of plankton, which are food for small fishes. Most fish species have a fairly narrow range of optimum temperatures related both to the species basic metabolism and the availability of food organisms that have their own optimum temperature ranges. In Indian EEZ, the mean surface temperature is about 29˚ C during summer (April- May). With the advancement of the summer monsoon (southwest monsoon) over the Arabian Sea and Indian Peninsula, the surface temperature cools down, reaching 25°C during August -September. The variations in the surface temperature are considered as an indiactor of fish abundance. The surface temperatre and ocean frontal zones, where temperature of water suddenly changes are used in locating the potential fishing zones using remote sensing techniques.
Thermocline
Remote sensing helps in locating the thermocline. In general, thermocline in the Indian sea is shallow during the southwest monsoon, moderate in summer and deeper during winter. Strong thermocline keeps the pelagic fishes like the coastal tunas and small pelagics above it and demersal fishes below it, thereby forming a natural barrier in between. During purse seining, shallow and strong thermocline make fishery very effective, especially when the vertical depth of the gear is more than the depth of the thermocline. The seawater temperature gradient is useful in commercial fisheries for

• Setting the depth of the long lines for tuna fishing
• Setting the depth of the drift gill nets for shark and seer fishing
• Determination of optimum depth of mid water trawling
• Deciding the operational pattern of the purse seines.

ii. Salinity
The monthly mean surface salinity varies from 32.5 ppt to 36.1 ppt. The northern region of the Arabian Sea (Mangalore to Ratnagiri) is favourable for the mackerel fishery, while the southern region for sardine fishery. The reason attributed is the sudden increase in salinity occurring northwards (north of 130 N) from the region off Mangalore during major part of the year. The higher salinity values in the northern regions are favorable for mackerel fishery.
iii. Upwelling
In Arabian Sea, upwelling starts at the southern tip of the west coast b the end of May or early June and propagates northwards with time. During upwelling, the oxygen minimum layer emerges from 100 m to 150 m depth to the surface, especially in the areas between Quilon and Kasargod. As a result, some fish population move in to the shallow surface waters while the others move offshore, away from the centre of strong upwelling. Pelagic fishes like mackerel, oil sardine and white baits avoid temporarily the areas of intense upwelling and concentrate into dense schools close to the surface and the coast in the near shore grounds, affording good catches. With the progress of monsoon and upwelling towards north, pelagic shoals also follow the trend and spawn intensely during June to September when there is maximum availability of larval food. The economic benefit of upwelling is the large concentration of commercially important fishes such as oil sardine, mackerel and white baits in those areas. During upwelling chlorophyll a, primary productivity and secondary productivity (zooplankton biomass) increase several folds.
iv) Primary and secondary production
Several attempts have been made to relate primary productivity to that of potential yield or optimum sustainable yield. From our EEZ of 2.02 million km², the total estimated production is 283 million tonnes of carbon. Therefore, the calculated harvestable fishery resource from our EEZ would amount to about 5.5 million tonnes (0.02% of the calculated primary production). Secondary production along the Indian coast ranges from 1.4 mg C /m²/day to 57.3 C /m²/day.
v) Estimation of chlorophyll fronts
Phytoplankton pigments form base in marine food chain and therefore, this parameter forms the direct link to plankton feeders. Detection of chlorophyll rich waters from satellite image ensures availability of food for fish and hence, positive indication of fish availability.