Ecosystem Targets and Supporting Indicators

Warm Water/Cold Water Fish Index

This indicator tracks the species composition of fish that prefer colder temperatures and those that prefer warmer temperatures.

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Fish Index (mean species/sample)
	Cold-Water Species (Spring Trawl)	Warm-Water Species (Spring Trawl)	Cold-Water Species (Fall Trawl)	Warm Water Species (Fall Trawl)
1984	7.1	6.1	4.1	6.9
1985	6.4	5.0	3.4	6.8
1986	6.3	4.7	3.9	6.8
1987	6.7	3.6	3.3	5.9
1988	6.8	4.0	4.4	7.3
1989	7.6	4.1	4.3	5.9
1990	7.7	4.2	4.2	6.7
1991	7.3	4.7	3.6	6.9
1992	6.9	4.7	4.6	7.1
1993	6.9	3.8	4.7	7.8
1994	6.1	4.6	4.7	7.6
1995	6.4	4.1	3.3	6.5
1996	6.3	4.8	4.5	8.4
1997	6.7	5.2	5.1	7.6
1998	6.8	5.2	4.0	7.8
1999	6.9	5.8	4.0	9.2
2000	6.4	6.2	3.8	8.9
2001	6.2	5.3	3.6	8.7
2002	6.3	7.5	3.3	9.9
2003	5.6	5.7	3.7	9.6
2004	5.9	5.2	3.6	8.8
2005	4.9	4.6	3.9	8.5
2006	4.7	5.8	2.7	7.2
2007	6.0	5.8	3.1	8.9
2008	6.0	5.9	4.1	9.7
2009	5.6	6.0	3.7	8.5
2010	6.1	5.3
2011	5.9	6.5	3.4	9.5
2012	5.9	8.0	3.1	11.3
2013	5.1	7.1	2.6	10.5
2014	4.6	6.8	2.8	10.5
2015	4.0	5.8	2.8	9.9
2016	4.5	7.6	1.7	9.8
2017	3.5	8.5	1.8	9.6
2018	4.5	7.5	2.3	11.0
2019	5.1	8.5	2.3	10.6
2020
2021	3.8	9.1	2.2	11.4
2022	3.7	7.1	2.3	10.7
2023	3.7	9.7	2.0	12.1

WHAT IS THE WARM/COLD SPECIES INDEX?

Finfish species captured in the CT DEEP Long Island Sound Trawl Survey were divided into adaptation groups based on their temperature tolerance and seasonal spawning habits. Members of the cold-adapted group (numbering 33 species including alewife, Atlantic cod, and winter flounder) prefer water temperatures below 15C (60°F), tend to spawn early in the year, and are more abundant north of the Sound than south of New York. In contrast, the warm-adapted group (currently numbering 46 species including striped bass, black seabass, and summer flounder) prefer warmer temperatures (11-22°C or 50-72°F), tend to spawn later, and are more abundant south of the Sound than north of Cape Cod. The index is the average number of species in each group (also called species richness) captured in spring (April, May, June) and fall (September, October). Survey samples each year from 1984 through 2023.

WHAT DOES IT INDICATE?

Climate change can disturb the species composition of an estuary such as Long Island Sound. A diverse community, that relies on different habitats and food resources, and is made up of some species that are generalists, and some that are specialists has a greater likelihood of withstanding disturbances than one with only a few species. A finfish community with high diversity (or species richness) indicates that the many different habitats in the estuary are productive. The number of species present, or species richness, can be an indicator of how healthy and diverse that fish community is. The ratio of cold-adapted to warm adapted species can provide insight on the effect of climate change on the finfish community if species tolerance to warming water temperature is considered.

STATUS

The trend in these indices shows that the average number of warm-adapted species captured in the survey both in spring and in fall has increased while the average number of cold-adapted species has decreased since 1984. All four trends (spring and fall warm and cold) are statistically significant, though the increase in warm water fish observed in the fall is the most dramatic because this is when water temperatures are highest. Many factors affect the number of species in the Sound and their abundance. And although overall finfish diversity in the Sound remains high, the composition of the finfish community is changing in favor of species tolerant of warming temperatures.