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Commercial Fisheries News
Volume 36 Number 1
September 2008
Higher water temps trigger ocean changes
Both fishermen and scientists have observed changes that signal important shifts in ocean conditions affecting the fisheries in our region. These changing conditions likely will have a more profound effect in the years to come.
Long-term records for the North Atlantic show periods of high and low temperature that can persist for decades. From a low point in the 1960s, we have seen an increase in temperature to a recent high, and climate models predict further increases (see Figure 1).
Increasing water temperatures are affecting the distribution of marine fish species in the Northeast. Analyses of 36 stocks by the National Marine Fisheries Service’s Northeast Fisheries Science Center (NEFSC) indicate that about one-third of them already are showing a significant northward shift.
Temperature changes in coming decades may make some areas uninhabitable for species such as cod that are at the southern extent of their range in our region, or reduce survival of the young in areas that may become marginally suitable because of increasing temperatures.
On the flip side, warmer waters also can mean that southern species will become increasingly important in Northeast fisheries. Atlantic croaker, an important recreational and commercial species in the Mid-Atlantic region, has both increased in abundance and expanded its distribution to the north in recent years.
Phytoplankton
These temperature changes also have implications for the basic productivity of the marine ecosystem. As temperatures increase, layers develop in the water column with warm water on top and colder water on the bottom.
This layering, called stratification, is fully developed each year in late spring. Stratification reduces the mixing of nutrient-rich bottom water with the shallower, sunlit waters where microscopic plants called phytoplankton thrive. It also can reduce the amount of food that ultimately reaches bottom waters to sustain bottom-dwelling fish and shellfish.
Before stratification is fully established, a spring “bloom” of phytoplankton usually occurs that starts the yearly production cycle. A second bloom often occurs in autumn as stratification begins to break down.
Recent work at the NEFSC has shown that a relatively strong fall bloom is related to strong haddock recruitment. In 2007, the fall bloom failed to develop (see CFN June 2008 page 26A), possibly signaling a difficult year for haddock recruitment.
Food for fish
Changes in physical conditions also have changed other parts of the food web. For example, long-term ecosystem monitoring by the NEFSC has shown changes in the abundance of different types of tiny crustaceans called copepods.
These species graze on phytoplankton and, in turn, are essential food for larval fish and other species, including some marine mammals such as right whales.
In the 1990s, there was a shift from larger copepod species to smaller ones in our region (see Figure 2). In the North Sea, similar changes have been related to less favorable conditions for larval cod survival.
Studies in both eastern Canada and the US also suggest that intensified stratification has fundamentally changed the ecosystem from one dominated by bottom-dwelling groundfish to one with large numbers of fish that occupy the water column, like herring and mackerel.
These changes are thought to be related to changes in temperature and stratification, affecting the flow of food and energy in the system as a whole.
Growth rates
We have noticed important changes in the growth rates of Northeast groundfish stocks. Of 20 stocks examined by the NEFSC in a recent analysis for the Groundfish Assessment Review Meeting (GARM), more than half have decreased in their average size or weight.
When fish stocks are at high abundance, the average size at age may decline because individuals compete for the same food. For example, the average weight of a two-year-old haddock on Georges Bank from the very large 2003 year class was eight-tenths of a pound in NEFSC fall surveys. Ten years earlier, a fish of the same age weighed more than twice as much – 1.8 pounds.
The average size of Georges Bank cod for older ages, on the other hand, has declined over the past 20 years even as the population has declined. This declining trend, also observed for cod and other stocks in Canadian waters, appears to be related to changes in the basic productivity of the system – possibly related to the quality of available food.
Changes in growth have important consequences for the biological targets and fishing limits called reference points that are used in fishery management. The most recent report to the New England Fishery Management Council (see CFN July 2008) suggested proposed revisions to biomass targets that were, in total, about 21% less than estimates derived in 2002.
Holistic perspective
Changes in fish stock distribution and abundance are not caused only by fishing or only by environmental conditions – they interact.
The NEFSC remains committed to the basic ecological research needed to understand this interaction in a way that is useful to fishermen and to fishery managers. As environmental conditions change with long-term climate trends, this focus has taken on increasing importance.
Our goal is to complement existing stock assessment advice and provide a holistic ecosystem perspective – one that looks at the entire system – to help guide management decisions in the future.
Northeast Fisheries Science Center
This article is the result of the combined efforts of a number of professionals at the National Marine Fisheries Service’s Northeast Fisheries Science Center in Woods Hole, MA. Contributors include: Elizabeth Brooks, Mike Fogarty, Teri Frady, Kevin Friedland, Jon Hare, Janet Nye, Loretta O’Brien, Paul Rago, and Ken Sherman.
An easy-to-read report on the ecology of the Northeast Continental Shelf and the government’s evolving ecosystem approach to fisheries management can be downloaded online at <www.nefsc.noaa.gov/ecosystems/Ecosystems.pdf>.
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