Ecosystem Targets and Supporting Indicators
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The Invertebrate Biomass Index is the annual average weight in kilograms of 15 invertebrate species collected in the Long Island Sound Trawl Survey. These species include various crab species (blue crab, flat claw hermit crab, horseshoe crab, lady crab, rock crab, and spider crab), American lobster, Lion’s Mane jellyfish, starfish, mantis shrimp, long-finned squid, gastropods (whelks and northern moon shells), and bivalves (blue mussel and common oyster).
The invertebrate biomass index indicates the productivity of the forage base of the food chain. Many game fish species, which are both recreationally and commercial valuable, depend on the success of invertebrate species as they are important food sources. Invertebrate species Like the finfish biomass, the high invertebrate biomass also indicates good health conditions of the Sound.
The Invertebrate Biomass Index shows a decreasing trend over the past 27 years. Although the American lobster makes up a large component of the invertebrate index early in the time series, analysis show a pronounced decline in many other invertebrate species over the same time series as well – such as spider, rock and lady crab species.
The invertebrate biomass indicators combine 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.