© 2006 International Council for the Exploration of the Sea
Interactions between striped bass (Morone saxatilis) rebuilding programmes and the conservation of Atlantic salmon (Salmo salar) and other anadromous fish species in the USA
New Hampshire Fish and Game Department, 225 Main Street, Durham, NH 03824, USA
e-mail: dgrout{at}nhfgd.org.
Anadromous populations of striped bass occur along the Atlantic coast of the US from Maine to North Carolina. Recruitment overfishing and declining water quality led to substantial reductions in striped bass abundance during the 1970s and 1980s. Cooperative interstate fishery management of striped bass began in 1981, with the development of a fishery management plan by the Atlantic States Marine Fisheries Commission, an organization of Atlantic coastal states. Effective fishery management and additional research and monitoring contributed to a tenfold increase in abundance of striped bass stocks by the late 1990s. This dramatic increase resulted in increased predation on a variety of anadromous fish species including American shad, blueback herring, and alewives. Predation by striped bass on Atlantic salmon smolts in North America has been documented, but the impact of this predation has not been quantified. Moderate to strong correlations were found between estimates of striped bass abundance and the return of Atlantic salmon to three of the four major New England salmon rivers. Further research is required to quantify the proportion of smolt production consumed by striped bass, particularly for salmon populations listed as endangered under the US Endangered Species Act.
Keywords: anadromous fish, Atlantic salmon, management, predation, striped bass
Received 13 October 2005; accepted 18 March 2006.
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Introduction |
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Anadromous populations of striped bass (Morone saxatilis) occur along the Atlantic coast of North America. Five spawning populations have been identified from Maine to North Carolina, with Chesapeake Bay and the Hudson River estuary contributing most to the Atlantic coast fisheries, followed by the estuaries of the Delaware, Roanoke, and Kennebec rivers (Berggren and Lieberman, 1978; Wirgin et al., 1993). Most anadromous striped bass spend the first few years of life in their natal estuaries before migrating to the coast to feed and over-winter (Merriman, 1941). Tagging studies indicate that striped bass from these populations undertake annual oceanic migrations during spring and summer, taking them as far north as Maine and southern New Brunswick, Canada (Figure 1), before returning south during the autumn to ocean areas near their natal spawning and nursery areas.
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Historically, striped bass have supported one of the most important commercial and recreational fisheries in the US north- and mid-Atlantic coasts. The recreational harvest peaked at 11 485 t in 2003, although annual commercial landings have been as high as 4997 t (ASMFC, 2004). The total economic value of the US Atlantic coast striped bass fishery was estimated at just less than US$7 billion (109) in 2003 (Southwick Associates, 2005).
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Management of striped bass |
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Before 1981, management of striped bass by Atlantic coast fisheries occurred at the state level through state laws or regulations. The earliest regulation directed at striped bass was promulgated in 1892, when New York State implemented a weekly, 24-h prohibition on the use of nets in the Hudson River during the spring-spawning period (ASMFC, 1990). By the 1940s, most of the states from Maine to North Carolina had some regulations pertaining to striped bass. Typically these included minimum size limits of 25–30 cm total length (LT) in the states south of New Jersey and 41 cm (LT) in northern states.
During the 1970s, it became apparent that these regulations were insufficient to maintain the resource. Annual commercial landings declined from an average of 4788 t in 1965–1974 to 2539 t in 1975–1980 (Figure 2). During this period, striped bass recruitment indices in Chesapeake Bay (Figure 3) and Roanoke River (Figure 4) also declined to their lowest recorded levels. Owing to the migratory nature of this species, cooperative management was needed to ensure that states with strong conservation measures would not lose the benefits to those with less restrictive management measures.
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In 1981, the Atlantic coast states, working through the Atlantic States Marine Fisheries Commission (ASMFC), developed the first cooperative interstate Fisheries Management Plan (FMP) for striped bass. The ASMFC, an organization of the 15 US Atlantic coast states, was established in 1942 by an act of the US Congress "to promote the better utilization of the fisheries, marine, shell, and anadromous, in the Atlantic seaboard". The plan's goal was to stop the decline in abundance of striped bass through increased minimum size limits and spawning area closures during the spawning season. Most states had adopted the recommended measures by 1984.
Over the next two years, the ASMFC implemented three amendments to the plan, encouraging states to enact further restrictions to reduce fishing mortality levels. The third amendment protected the 1982 year class until 95% of the females had spawned at least once. This year class was only of average abundance, but was the strongest since the plan was implemented. To protect the year class, minimum size limits were gradually increased to 97 cm (LT) in 1985–1990. In addition, the states of Maryland and Delaware declared moratoria on the harvest of striped bass in 1985. These measures protected the 1982 year class, as well as subsequent weaker year classes from exploitation.
The adoption of the Atlantic Striped Bass Conservation Act by the US Congress in 1984 was another important milestone in the efforts to restore striped bass. Before this, implementation of various management measures in the FMP was at the discretion of individual states, because the ASMFC did not have direct regulatory authority over state fisheries. The Act strengthened the ASMFC's ability to manage striped bass by stipulating that, if a state did not comply with the FMP, the ASMFC could request the US Secretary of Commerce to impose a federal moratorium on striped bass fishing in that state. Two other pieces of federal legislation, the Emergency Striped Bass Study and an amendment to the Anadromous Fish Conservation Act, provided funding for science, research, and monitoring of striped bass that was crucial to sound management.
In 1989, juvenile striped bass surveys in Chesapeake Bay revealed strong recruitment for the first time since 1970 (Figure 3). This initiated the rebuilding phase of management, with the adoption of this to the FMP in 1990 (ASMFC, 1990). Amendment 4 established fishing mortality rate (F) targets of 0.25 for the rebuilding phase and 0.50 for stock maintenance, with restrictive commercial quotas of 20% of the average landings in 1972–1979, one or two fish per day recreational creel limits, and a two-tiered size limit of 51 cm (LT) for the "producer areas" of Chesapeake Bay, Delaware Bay, Hudson River, and Roanoke River, and 71 cm (LT) in all other areas from Maine to North Carolina. The two-tiered size limit was implemented to preserve historical allocation of landings because evidence suggested that the migration patterns of striped bass brought few individuals >71 cm (LT) to the producer areas, except during the spring-spawning run. Most states voluntarily maintained more restrictive regulations than those in Amendment 4.
In 1995, the ASMFC declared the Chesapeake Bay stock of striped bass fully recovered. Three years later, the ASMFC determined that the Delaware River and Roanoke River stocks had also recovered. It should be noted that fisheries dependent and survey data from the Hudson River, indicated that this stock did not decline in abundance as other stocks had during the 1970s and 1980s. During the five-year restoration phase under Amendment 4 (1990–1994), fishing mortality rates remained close to the target of 0.25, adult-abundance indices continued to increase, and estimates of spawning-stock biomass (SSB) approached 1960s levels (ASMFC, 1998). Juvenile recruitment indices were above average in 1992–1995, and in 1993, were the highest recorded in the 40-year time-series (Figures 3 and 4).
Amendment 5 was adopted in 1995 to address management of the recovered stocks. The target fishing mortality rate was set at 0.33, halfway between the earlier target of 0.25 and the rate of maximum sustainable yield (0.40). It maintained the two-tiered size limits of 51 cm and 71 cm (LT) as preferred minimum sizes. However, the FMP provided states with many additional options for management through the use of bag limits, seasonal restrictions, and quotas under varying minimum sizes (ASMFC, 1995).
Following the adoption of Amendment 5, the total abundance of striped bass stocks stabilized for several years before increasing further in recent years (Figure 5). Recruitment indices showed that dominant year classes were produced in 1996, 2000, 2001, and 2003 in Chesapeake Bay (Figure 3), and the SSB reached a peak in 2001 (Figure 5). The recreational catch and harvest reached a peak in 2003, while commercial landings peaked in 1998, but subsequently returned to lower levels (Figure 2). The recreational fishery continues to account for the largest share (75%) of the total annual losses, harvest and discard mortality combined (ASMFC, 2004).
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Management of striped bass was further refined in 2003 with the adoption of Amendment 6 which contains, among other things, specific targets and thresholds for F and female SSB, as well as rules mandating changes in management if targets and thresholds are violated (ASMFC, 2003). In addition, commercial quotas were increased to 1972–1979 landing levels.
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Striped bass interaction with other anadromous fish species |
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The successful management programme for striped bass resulted in an approximately tenfold increase in abundance during the 1980s and 1990s (Figure 5). This dramatic increase resulted in greater predation pressure on a variety of fish and invertebrates, including anadromous fish species. Hartman (2003) used a bioenergetics model to estimate that the annual coast-wide consumption by striped bass increased eightfold, from 17 900 t to 147 000 t in 1982–1995. Although striped bass have been reported to be generalist feeders, a variety of studies have shown that anadromous species, such as blueback herring (Alosa aestivalis), alewives (Alosa pseudoharengus), and American shad (Alosa sapidissima) can make up a substantial proportion of their diet in many areas (Trent and Hassler, 1966; Drew, 1988; Nelson et al., 2003; Walter et al., 2003; Savoy and Crecco, 2004). The shared migration pattern of striped bass and these other anadromous species brings them together during spawning (Walter et al., 2003). In addition, non-spawning, migratory striped bass have been observed feeding on river herring during their spring-spawning migration in New England rivers, including the Connecticut River (Savoy and Crecco, 2004), the Merrimack River (J. Mckeon, pers. comm.), and several rivers in New Hampshire (C. Patterson, pers. comm.) and Maine (T. Squires, pers. comm.).
Savoy and Crecco (2004) used statistical and empirical evidence to support the predation hypothesis as a direct link between increased striped bass abundance and the decline in adult American shad and blueback herring abundance in the Connecticut River during the 1990s. The decline of alosines in this river was substantial, with annual shad abundance declining from >1 million in 1991–1992 to approximately 300 000 in 1994–1995. The decline in the annual abundance of blueback herring was even more significant, falling from 300 000–600 000 in 1981–1992 to 2000–10 000 per year in 1999–2002. The declines in alosines occurred despite above-average juvenile abundance indices for most years during the 1980s and early 1990s and coincided with in-river population estimates for striped bass of 400 000–1.7 million annually, during the months of April, May, and June.
The movement of striped bass into coastal rivers in New England during the spring also coincides with the downstream migration of Atlantic salmon (Salmo salar) smolts during April and May (Moring et al., 1995). Although Atlantic salmon have not been reported to be the main dietary component of striped bass in any feeding study (Walter et al., 2003), predation by striped bass on Atlantic salmon smolts has been documented in the Narraguagus River (Beland et al., 2001) and the Merrimack River (Blackwell and Juanes, 1998). The latter study documented extensive striped bass predation on Atlantic salmon smolts below a hydroelectric dam, with smolts being consumed by more than 48% of the striped bass that contained prey, and comprising more than 80% of the total mass of prey remains.
Adult returns of Atlantic salmon to US rivers have declined recently. Total Atlantic salmon returns peaked at 5624 in 1986 and subsequently declined to a low of 803 in 2000 (USASAC, 2005). In 2000, the Atlantic salmon populations in the eastern rivers of Maine were listed as endangered under the US Endangered Species Act (USASAC, 2002).
The decline in Atlantic salmon abundance was concomitant with the increase in striped bass abundance along the east coast. With empirical evidence of striped bass predation on Atlantic salmon smolts, a statistical comparison of striped bass abundance with adult salmon returns from selected New England rivers was conducted to determine if a correlation existed between the two trends. Since in-river measurements of striped bass abundance were only available for the Connecticut River, coast-wide estimates of striped bass abundance in 1982–2002, developed by an ADAPT virtual population analysis model (Figure 5 and ASMFC, 2004), were compared with Atlantic salmon returns, lagged by two years to the Connecticut, Merrimack, Penobscot, and St Croix rivers. These rivers were chosen because they had a complete time-series of adult returns during these years, and annual returns were relatively large, i.e. several hundred fish in most years. Total annual returns were used because two-sea-winter fish comprise most returns in these rivers (USASAC, 2002). The results of the correlation analysis varied, with the Penobscot and St Croix rivers showing strong negative correlations of –0.80 and –0.84, respectively, while the Merrimack and Connecticut rivers showed relatively poor correlations (Tables 1 and 2).
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Savoy and Crecco (2004) developed estimates of in-river abundance of striped bass in the Connecticut River in 1994–2002 by using population estimates from a 1994 tagging study and scaling the results to the mean catch-per-tow for striped bass in Long Island Sound from the Connecticut Department of Environmental Protection trawl surveys during those years. When these estimates were compared with adult salmon returns two years later, a moderately strong negative correlation of –0.66 was observed (Table 2).
Although a statistical correlation is not in itself evidence of cause and effect, it does suggest that, when combined with empirical evidence, the nearly tenfold increase in the coast-wide striped bass abundance may be having a negative effect on Atlantic salmon conservation and restoration efforts. Studies from the Pacific coast of North America indicate that significant predation by striped bass on salmonids is possible. Johnson et al. (1992) estimated the annual salmonid smolt losses from striped bass predation in Coos Bay, Oregon to be 41 000–383 000, with striped bass abundance at 17 382–43 409. Even relatively low losses of Atlantic salmon smolts from predation by striped bass could be significant, particularly in New England rivers with critically small salmon populations.
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Conclusions |
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TopIntroductionManagement of striped bassStriped bass interaction with...ConclusionsReferences |
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Effective cooperative interstate management has resulted in an approximately tenfold increase in the abundance of striped bass on the Atlantic coast of North America, since the 1980s. Restoration of striped bass has allowed increased commercial harvests and enhanced recreational fishing opportunities that have contributed greatly to the economy of the US Atlantic coastal states. On the other hand, the rise in abundance has resulted in increased predation on other species of fish and invertebrates. However, because striped bass are generalist feeders and undertake extensive annual migrations, the estimated eightfold increase in predation demand is spread across a wide variety of species, over an extensive area of the coast. These life history characteristics should limit the impact of striped bass predation on many species, but Savoy and Crecco (2004) and Johnson et al. (1992) have demonstrated the potential for striped bass to have a significant impact on local populations of anadromous alosines and salmonids. Recent persistent declines in Atlantic salmon returns to New England rivers have been concurrent with the increased abundance of striped bass. Predation by striped bass on Atlantic salmon smolts has been documented in at least two New England rivers, and statistical analysis shows moderate to strong negative correlations between estimates of striped bass abundance and Atlantic salmon adult returns in three of the four major Atlantic salmon rivers in New England. Given the critically low abundance of Atlantic salmon in many New England rivers, the increased striped bass abundance could possibly hinder Atlantic salmon conservation and restoration efforts.
With this in mind, further research should be undertaken to quantify the proportion of smolt production consumed annually by striped bass, particularly for those eastern Maine salmon populations listed under the Endangered Species Act. Such studies would provide fishery managers and scientists with a better understanding of how much striped bass predation contributes to the critically low levels of Atlantic salmon currently observed in the US.
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Acknowledgements |
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I extend special thanks to Joshua Moser for creating the map of tag returns and Gary Shepherd for his review and comments on earlier versions of this manuscript.
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References |
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