Biological response to changes in climate patterns: population increases of gray snapper (Lutjanus griseus) in Texas bays and estuaries.(Report)

Abstract--The increase in the abundance of gray snapper (Lutjanus griseus) in Texas bays and estuaries over the past 30 years is correlated to increased wintertime surface water temperatures. Trends in the relative abundance of gray snapper are evaluated by using monthly fishery-independent monitoring data from each of the seven major estuaries along the Texas coast from 1978 through 2006. Environmental conditions during this period demonstrated increasing annual sea surface temperatures, although this increase was not seasonally uniform. The largest proportion of temperature increases was attributed to higher winter temperature minimums since 1993. Positive phases of the North Atlantic Oscillation, resulting in wetter, warmer winters in the eastern United States have occurred nearly uninterrupted since the late 1970s, and unprecedented positive index values occurred between 1989 and 1995. Increases in water temperature in Texas estuaries, beginning in the early 1990s, are postulated to provide both favorable over-wintering conditions for the newly settled juveniles and increased recruitment success. In the absence of cold winters, this species has established semipermanent estuarine populations across the entire Texas coast. A shift to negative phases of the North Atlantic Oscillation will likely result in returns to colder winter temperature minimums that could reverse any recent population gains.

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Decadal and multidecadal climate variability in the eastern United States and North Atlantic Ocean are sensitive to natural climate variability associated with the North Atlantic Oscillation-Arctic Oscillation (NAOAO; Okumura et al., 2001). Ocean-atmosphere interactions between sea-level atmospheric pressure and sea surface temperature have resulted in periods of intensified climate variability at 50-80 year cycles (Enfield et al., 2001; Cronin et al., 2003). Inter-annual fluctuations in fishery catches that are linked to climate variability occur in many marine systems; the best examples are found in the North Pacific Ocean and Bering Sea (reviewed in Roessig et al., 2004, but also see Perry et al., 2005; Zeeburg et al., 2008). Huge increases in fish populations in the North Atlantic Ocean ("fish outbursts" in Hare and Able [2007]) have been linked to decadal-scale fluctuations in both atmospheric pressure gradients and water temperatures. Examples include dramatic fluctuations in gadoids (Sirabella et al., 2001), sciaenids (Hare and Able, 2007), pleuronectids (Sullivan et al., 2005), and in the size of fish assemblages--the latter caused by changes to the estuarine ecosystem (Attrill and Power, 2002). Changes in fish populations along the west coast of the Americas have also been related to shorter time-scale climate variation associated with El Nino patterns (Arcos et al., 2001; Smith and Moser, 2003). Few studies have examined the link between ecosystem structure, fisheries production, and climatic forcing along the southeastern coasts of the United States (including "the Gulf" of Mexico, hereafter referred to as "the gulf"), but those that have (Parker and Dixon, 1998; Sullivan et al., 2005) generally identify warmer winter temperatures as the cause of distributional changes in fish populations (Oviatt, 2004; Preston, 2004).

The gray snapper (Lutjanus griseus) is a temperate and tropical reef fish, commonly found in marine and estuarine waters in the western Atlantic Ocean from Florida through Brazil, including Bermuda, the Caribbean, and the northern Gulf (Randall, 1968; Rutherford et al., 1989a). Although the young of this species have been collected as far north as Cape Cod, Massachusetts (Denit and Sponaugle, 2004), they are most prevalent in continental and island shelf waters in the vicinity of south Florida, Cuba, and Venezuela (Allman and Grimes, 2002). Larvae and juvenile fish are common inshore in seagrass and mangrove nursery areas and have been even found in freshwater estuaries. Adults are usually associated with complex structures (such as rocky areas, coral reefs, pilings, and docks) and mangrove sloughs (gaining them the often-used local name of mangrove snapper). Because of their affinity for hard bottom substrates, gray snapper are especially common on the numerous artificial reefs, such as oil and gas platforms, and on other man-made structures located on the northern gulf continental shelf from Alabama to Texas (Fischer et al., 2005).

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Studies of the life history characteristics of gray snapper (Rutherford et al., 1989b; Burton, 2001) have revealed this species to be both euryhaline (they have been observed in salinities ranging from 0 to 67) and thermally tolerant (in temperatures ranging from 12.8[degrees] to 31.7[degrees]C). The lower lethal temperature limit is reported to be 11-14[degrees]C (Starck and Schroeder, 1971) and increased mortalities accompany sudden temperature drops. Cold kills of gray snapper (along with other estuarine and coastal fishes) have been reported at numerous times along the Texas Gulf coast (Gunter, 1941, 1951; McEachron et al., 1994). These cold kills of tropical species in Texas waters have previously been associated with warmer than usual summer and fall temperatures that stimulate tropical species either to move inshore or to remain inshore much later into the winter than would otherwise occur, thereby leaving them more vulnerable to sudden cold spells (Moore, 1976, Holt and Holt, 1983).

Texas Parks and Wildlife Department (TPWD) has used various gears systematically in Texas estuaries since 1975 to assess changes in the abundance and size of organisms, their spatial and temporal distributions, species compositions of the community, and selected environmental parameters known to influence their distribution and abundance. The purpose of this study was to examine the relationship between water temperature and population trends in juvenile and adult gray snapper within the seven major estuaries along the Texas gulf coast.