Ecosystem Targets and Supporting Indicators
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The Fish Biomass index is the annual average weight in kilograms of all of the finfish species collected in the Long Island Sound Trawl Survey (about 50-70 different species per year). The index does not include invertebrates taken in the Survey. It is designed to reflect the total finfish biomass supported in the Sound.
Species richness is the number of species within a biological community (i.e., Long Island Sound). This indicator represents the average number of finfish species caught in each tow.
The fish biomass index is an indicator of fisheries productivity in Long Island Sound. High fish biomass suggests that conditions in the local estuarine environment are good, including factors such as water quality, habitat, and the abundance and diversity of available food. Species richness measures the diversity of species supported within the Sound’s various habitats.
The Fish Biomass index has fluctuated over the past 27 years but has not shown any up or down trend. This stability indicates that while the abundance of individual species may have increased or decreased, the estuary has maintained its overall rate of finfish production. Species with the highest or lowest biomass have changed over this period. For example, butterfish was among the largest contributors by weight for several years. High numbers of scup were largely responsible for the record year for fish biomass in 2002. Successful fishery management efforts have contributed to increased abundance of many managed species. Additionally, some species may be increasing because they are tolerant of steady increases in water temperature seen coast-wide, while others may be declining in the Sound because they cannot tolerate our warmer waters.
With species richness, the high and stable number of counts per tow indicates that the Sound is healthy and that a strong balance of species is able to exploit the full mix of resources available throughout this ecosystem.
The Fish Biomass indicator combines data from both the spring and fall trawl surveys. Numbers of each species captured in each Survey tow are converted to the log scale in order to de-emphasize extraordinarily large or small catches when calculating an average catch for the year. This log-scale mean is then re-converted to the regular arithmetic scale and referred to as a geometric mean. In a natural environment such as Long Island Sound fish have a “patchy” distribution, i.e. some areas will have a very high abundance and other areas very low abundance or no fish at all. A simple arithmetic mean can be easily biased by any unusually high or low values and will not always reflect the true center of a data set.