Ecology and Vulnerability

Winter Flounder

A growing number of studies provide strong evidence that marine fish and invertebrate species are responding to climate change through geographical range (Nye et al., 2009) and phenology shifts (Staudinger et al., 2019; Langan et al., 2021). Studies of range shifts generally use long-term federal datasets from spring and fall bottom trawl (NOAA Fisheries 2022) and ichthyoplankton Ecosystem Monitoring (EcoMon) (Walsh et al., 2015) surveys of the continental shelf, or by state and private surveys of regional bays and estuaries (Morson et al., 2019, Snyder et al., 2019; Langan et al., 2021). These extensive monitoring programs have shown that centers of population biomass for the majority of marine fish species are shifting poleward towards higher latitudes and to greater depths at rates that exceed terrestrial species (Lenoir et al., 2020). These general trends are projected to continue under future climate scenarios of modeled habitat abundance (Kleisner et al., 2017), suitable thermal habitat (Morley et al., 2018), and relative population biomass (Allyn et al., 2020). 

An analysis of 42 species of marine fish and invertebrates (RSGCN and non-RSGCN species) found in continental shelf waters of the Northeast region between 1974 and 2022, showed the community has shifted on average 0.75 degrees (83.83 km) north during spring, and 0.61 degrees (68.27 km) north during fall. The community also experienced a change in average depth of 22.4 meters and 8.6 meters deeper in spring and fall, respectively (NOAA Fisheries 2022). Similar to patterns observed at the community scale and for non-RSGCN species, the direction, rate, and magnitude of range shifts varies widely among RSGCN marine fishes and elasmobranchs sampled during this period; responses in range changes span -0.51 km southward in Winter Skate, to 2.25 km northward in Black Sea Bass during spring, and -0.21 km northward in Atlantic Herring to 2.05 km northward in Black Sea Bass during fall. Depth range shifts spanned from -61.08 meters deeper in Atlantic Herring to 12.41 km shallower in Atlantic Cod in spring, and -50.88 meters deeper in Atlantic Herring to 31.20 meters shallower in Atlantic Halibut during fall (NOAA Fisheries 2022). 

Shifts in phenology are more difficult to track in comparison to range shifts in aquatic species because observations need to have repeated observations within a narrow and set area over multiple decades (Staudinger et al., 2019). Several multi-species analyses have shown phenological shifts in the occurrence of larval phases of demersal groundfishes (Walsh et al., 2015), growth and productivity of population biomass (Henderson et al., 2017), as well as seasonal migration patterns (both ingress and egress) and residence times in estuaries systems such as Narragansett Bay, Rhode Island (Langan et al., 2021).

As species make distributional changes in range and phenology, primarily in response to warming sea surface and bottom temperatures, entire assemblages of marine species are reshuffling and reorganizing across Northeast coastal ecosystems (Weiskopf et al., 2020; Staudinger et al., 2021). Examples of changes in community structure through altered species diversity, population size, and density have been documented across the Northeast continental shelf (Friedland et al., 2020) and in estuaries including Little Egg Inlet, New Jersey (Morson et al., 2019), Narragansett Bay, Rhode Island (Langan et al., 2021), and Long Island Sound off of New York and Connecticut (Snyder et al., 2019). Ensembles of warm-water associated species that are moving north into temperate habitats are generally replacing cold-water associated species, which are moving to deeper depths and higher latitudes in search of cold water refugia (Friedland et al., 2020; Pershing et al., 2021).

On a global scale, warming has led to a decrease in body size (Sheridan and Bickford 2011) but this varies widely by species and local conditions. The indirect effects of morphological and behavioral changes combined with shifting distributions is changing predator-prey and competitive relationships among associated marine fish species and resulting in novel trophic interactions (Staudinger et al., 2021). Most models that project range shifts currently do not account for species interactions (Pearson and Dawson 2003, MacPherson et al., 2021); this is an area of active research as changes in trophic relationships are hypothesized to be more important for species survival under future conditions than environmental conditions (Louthan et al., 2015, Staniczenko et al., 2017). In addition, the combined stress from warming ocean temperatures, decreased dissolved oxygen, and ocean acidification is increasing the prevalence and vulnerability to disease and predation.

Shifts in Range, Elevation, or Depth

Between the periods of 1974-1977 and 2019-2022, Winter Flounder shifted its spring range across the Northeast continental shelf by 0.67 degrees (74.75 km) north and contracted its range by 0.8 degrees (89.48 km). Its fall range also shifted by 0.34 degrees (38.15 km) north and contracted by 1.85 degrees (205.08 km). During spring, Winter Flounder shifted to deeper waters by 16 meters and 11.3 meters deeper during the fall (NOAA Fisheries 2022). In inshore waters of the Chesapeake Bay, Winter Flounder shifted their range out of the area and moved north as winter temperatures have increased (Bell et al., 2018).

Winter Flounder is projected to experience significant losses of relative biomass during spring and fall by 2050 across the northeast continental shelf with larger decreases in the Southern New England – Mid Atlantic compared to the Gulf of Maine under mean model projections using the RCP 8.5 scenario (Allyn et al., 2020). There is low certainty that Winter Flounder is projected to shift the centroid of their range across the Northeast continental shelf by 96.3 km under RCP 2.6 and medium uncertainty they will shift 394.4 km under RCP 8.5 by the end of the century (2081-2100), with projected losses of available thermal habitat of -18.9% under a RCP 2.6 and -68.9% under RCP 8.5 when compared to the period of 2006-2020 (Morely et al., 2018).

Shifts in Phenology

The seasonal timing of occurrence of larval Winter Flounder shifted earlier during winter in the Southern New England to Gulf of Maine regions between the periods of 1977-1987 and 1999-2008 (Walsh et al., 2015). From 1972 to 2017, peak Winter Flounder larval density in Mount Hope Bay in Rhode Island shifted approximately 2 weeks earlier and was strongly correlated with temperature (McManus et al., 2021).

Changes in Population

Since the 1990s, Winter Flounder have experienced progressively lower levels of recruitment due to warming winter temperatures and shorter winter duration in East Coast estuaries, which has decreased the available habitat and refuge from predators for young flounders (Bell et al., 2014), though not all studies found a connection between changes in temperature and Winter Flounder decline (see Frisk et al., 2018). Future population models that take into account increasing winter temperatures also confirm reduced recruitment of early life stages of Winter Flounder due in part to higher natural mortality rates (Bell et al., 2018) resulting from greater spatial and temporal overlap with predators (Howell et al., 2016, McManus et al., 2021, Langan et al., 2022). Concerns have been raised by multiple authors that declines in recruitment and productivity are severe enough that even prohibiting fishing of Winter Flounder may be insufficient to restore their populations to historic levels (Bell et al., 2014, Langan et al., 2022).

In the southern part of their range off the coast of New Jersey, data from 1980 – 2010 indicated that while climate change played an indirect role in decreasing recruitment, a critical springtime temperature threshold affected population dynamics by increasing larval metabolism rates; higher metabolism is associated with higher and unsustainable feeding demands, especially in conjunction with increased predation pressure (Able et al., 2014). The abundance of Winter Flounder has decreased steadily in Norwalk Bay, Connecticut in the Long Island Sound between 1990 and 2016 in association with increasing water temperatures and hypoxic conditions (Crosby et al., 2018). These results are consistent with laboratory and simulation studies that have shown depressed recruitment and population biomass in response to higher dissolved CO2 combined with increasing water temperatures (Huebert et al., 2021).