Changing Ocean Temperatures Holds The Fate Of Food Supply To Young Atlantic Cod

Kelvin Friedland and colleagues, who are lead researchers in National Oceanic and Atmospheric Administration (NOAA), studied the distribution and the profusion of some important species of zooplankton, water temperatures of sea surface, and cod plenteousness. In their study, especially in the areas where stocks of Atlantic cod have had struggles in surviving after a period of over-fishing, the researchers found out that the abundance of zooplankton species that are critical in the survival of the cod larvae have reduced.

Friedland was quoted saying that the growth, reproduction and the ultimate distribution of the marine organisms is majorly governed by temperature. He further reported that the redistribution of these plankton communities on basin-wide and regional scales are caused by continuous shift in temperature distributions, which could either be triggered by human or natural factors. Friedland is the lead author in the study based on North Fisheries Science Center (NEFSC) which is a branch of NOAA.

A study in the press in the Progress in Oceanography journal has revealed that biological consequences that is evident in several levels of the food chain is as a result of continuous alterations in thermal habitats.

Friedland said that, ” The geographic and depth distributions of fish and shellfish populations can also change based on their preferred thermal habitats.” He added that future alterations in thermal conditions may lead to more shifts in the distribution of species through loss and gain of the local populations.

These group of scientists found out that there has been an increase in the temperatures of the ocean waters in the past decades in the regions of Northeast Continental Shelf. They realized however that these changes in the temperatures have not been equivalent in the entire ecosystem. Habitats with warm water were reported to have increased in temperature whereas habitats with cool water were reported to have dropped in temperature. The coldest habitats which have temperatures of between 1-4 C or 34-39 F either maintained their temperatures or had an increase in temperature slightly. This study was carried out between 1982 and 2011 with areas with temperatures 16 to 27 C or 60-80 F referred as warm habitats while those with temperatures of between 5 and 15 C or 41-59 F taken as cold habitats and which historically are taken to be the core habitats of the ecosystem. This discontinuity in temperature changes has been attributed to alteration in the Northern Gulf of Maine’s circulation that is associated with Labrador Current.

Atlantic cod are managed as two units of stock in the Northeast U.S. These two units are the more southerly cod stock located on and around Georges Bank and the more northerly cod in the Gulf of Maine. These two stocks houses upward of ten spawning populations known. The young fish produced by these two stocks are only found in specific areas of larval development. The Atlantic cod larvae feed on two types of zooplankton species namely Pseudocalanus spp which is fed on by winter-spawning cod and Centropages typicus which is fed on by spring-spawning cod.

Friedland and his crew developed a study of zooplankton and cod plenteousness in six index areas, which related to larval feeding and their development areas to the areas where adults resided on. Their study found out that Pseudocalanus spp has reduced in their number around Georges Bank and eastern Gulf of Maine. These are the areas where the Atlantic cod had reduced in the recent years. They also realized that Centropages typicus reduced in their numbers in the Eastern Gulf of Maine also. These areas where the zooplankton species have reduced are related to the alterations that occurred to the main thermal habitat of the ecosystem.

The profusion of zooplankton species is always measured by use of NEFSC’s Ecosystem Monitoring Program (EcoMon), that conducts shell-wide surveys of the ecosystem twice every month. The researchers also analyzed observations and data from surveys of the spring bottom trawl of the NEFSC.

Even though these scientists only concentrated on the Atlantic cod, their recruitment success and its population trends, these zooplankton species that were examined are an important prey for the early life stages of many other fish species as well as other marine mammals. The zooplankton species studied are very much important in the Northeast Shelf ecosystem, therefore, they can be used as a model organisms. The effect that thermal habitat has on the changes within the food web can be tested with this model.

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