The status of the southern New England lobster stock.
American lobster (Research)
Population declines (Control)
|Publication:||Name: Journal of Shellfish Research Publisher: National Shellfisheries Association, Inc. Audience: Academic Format: Magazine/Journal Subject: Biological sciences; Zoology and wildlife conservation Copyright: COPYRIGHT 2012 National Shellfisheries Association, Inc. ISSN: 0730-8000|
|Issue:||Date: June, 2012 Source Volume: 31 Source Issue: 2|
|Topic:||Event Code: 310 Science & research|
|Geographic:||Geographic Scope: United States Geographic Code: 1USA United States|
ABSTRACT Management of the American lobster fishery in U.S. waters
recognizes 3 biological stocks. Since 2001, the northern 2 stocks have
increased in abundance whereas the southernmost stock has declined
dramatically. Decline in abundance indices of all sizes, including
larvae and young-of-year, indicate that the stock is experiencing
recruitment failure. Increasing water temperature and a corresponding
increase in shell disease may be contributing factors. Assessment
procedures have recently included a 2-fold increase in nonharvest
losses; however, modeling spawning stock losses specific to the disease
process has not been accomplished. Rebuilding strategies need to
maximize stock production while at the same time minimize the spread and
severity of shell disease.
KEY WORDS: American lobster, Homarus americanus, southern New England, management
The American lobster (Homarus americanus, Milne Edwards) fishery has been an important component of New England's economy and culture for hundreds of years. Management of this fishery in U.S. state waters is under the jurisdiction of the Atlantic States Marine Fisheries Commission (ASMFC), whose management board recognizes 3 biological and geographical stock areas (Fig. 1): the Gulf of Maine (GOM), Georges Bank (GBK), and southern New England (SNE). Each area has an inshore and offshore fishery component, with the inshore fishery dominating in the GOM and SNE stocks, and the offshore fishery dominating in the GBK stock. Coastwide, the lobster fishery has historically exerted a high removal rate--or exploitation--on all 3 stocks. Total U.S. landings were relatively constant at about 14,000 mt from the 1960s through the late 1970s (Atlantic States Marine Fisheries Commission 2009) (Fig. 2). Landings more than doubled from 1980 to 2000, reaching a high of 42,500 mt in 2006, and leveled out between 35,000 mt and 40,000 mt in 2007 to 2009. This recent increase was primarily a result of record-high landings from Maine state waters.
STATUS OF THE THREE U.S. STOCKS
Historically, abundance and landings trends for the 3 U.S. stocks were fairly similar; however, this similarity started to diverge during the 1990s, and the current status of the 3 stocks is very different. The GOM stock supports the largest fishery, and landings from this stock have increased dramatically since 1990 (14,600-33,000+ mt). Reference GOM stock abundance, estimated annually using the University of Maine Assessment Model (Atlantic States Marine Fisheries Commission 2009), was at record high levels during the past 5 y compared with earlier years in a 26-y time series (1982 to 2007; Fig. 3A). Exploitation has remained steady for the GOM stock because recruitment of just-legal-size lobsters to the fishery has increased steadily since 1997. The GBK stock supports a much smaller fishery, but also experienced increased landings (1,3002,400 mt), and a large increase in reference abundance from 2001 to 2008 (Fig. 3B). Exploitation by this small fishery has declined during the same 26-y time series (1982 to 2007) because abundance increased much faster than harvest removals. Conditions in the SNE stock, which contributed about 20% to the total U.S. landings up until the 1990s, are quite the opposite of the 2 northern stocks. Although landings showed an increase from the early 1980s into the 1990s as with the other stocks, reaching a time series high of nearly 10,000 mt in 1997 to 1999, the fishery experienced a dramatic decline in landings to less than 3,000 mt from 2003 to 2007. Reference abundance declined to below median levels in 2001, to below the 25th percentile in 2002, and remained there through 2007 (Fig. 3C). Abundance below the 25th percentile is the trigger for management action to rebuild the stock (Atlantic States Marine Fisheries Commission 2009).
Several research trawl surveys occur in SNE waters and have provided consistent signals of abundance since the early 1980s. Trends in state (inshore) and federal (offshore) SNE fall abundance indices (delta mean catch per tow) for recruit (1 molt size below legal minimum) and legal sizes all generally peaked during the 1990s, and then declined to well below median levels thereafter. Legal indices remained at or below the 25th percentile of the long-term (1984 to 2003) abundance from 2002 to 2008 (Fig. 4). The only exception is Rhode Island Survey indices, which increased modestly from 2003 through 2008 after declining consistently with the other SNE indices. That state's extensive V-notch program and other measures, initiated in 2001 to 2006 as mitigation for a 1996 oil spill (Gibson & Angell 2006), apparently allowed for a significant increase in the adult population. Unfortunately, the increase in the population was short-lived; the 2009 trawl survey recruit and legal abundance indices were at or below the 25th percentile limit. Abundance indices for inshore western data (inshore New Jersey and western Long Island Sound (LIS)) showed the greatest contrast through the time series, with indices increasing higher and declining more quickly and more severely. However, none of the recent regional indices indicate any recovery of the stock.
EVIDENCE OF RECRUITMENT FAILURE IN THE SNE STOCK
The 2009 Atlantic States Marine Fisheries Commission (ASMFC) stock assessment (Atlantic States Marine Fisheries Commission 2009) concluded that the SNE stock was critically depleted, with nearly a decade of abundance less than the 25th percentile of the long-term (1984 to 2003) value. Since the release of the 2009 assessment, additional evidence was reviewed, and is presented here, that showed the reproductive potential and abundance of the SNE stock continued to decrease. Taken together, the data strongly indicate that the stock is experiencing recruitment failure caused by a combination of environmental drivers and continued fishing mortality (Atlantic States Marine Fisheries Commission 2010a). For management purposes, recruitment failure is defined as the point when environmental conditions and/or harvest removals result in successive years of such poor production of immature (subharvest size) animals that the stock is unlikely to sustain itself.
[FIGURE 1 OMITTED]
Environmental conditions play a large role in recruitment regardless of parent stock size, and unfavorable conditions put the stock at higher risk of failure. Multiple postlarval and young-of-year (YOY) indices (Figs. 5 and 6) are generated to monitor annual larval production and settlement in the SNE population. Larval production and settlement are inherently variable; however, past patterns of periodic strong year classes have been replaced with sustained poor production. Such persistent low production can only lead to reduced recruitment of harvest-size animals to the fishery and lower future abundance levels if mortality factors remain unchanged. It should be well noted that even though adult abundance from 2000 to 2009 was low but not out of historical ranges (i.e., reference population size of 10-15 million, Fig. 3C), prior low-abundance periods were accompanied by moderate to high larval and YOY indices. The joint condition of both adult and larval/YOY abundance at sustained low levels is strong evidence of recruitment failure.
IMPEDIMENTS TO REBUILDING
It is clear that SNE stock abundance is declining in the face of declining or stable fishery removal rates, making a compelling case that a substantial increase in the rate of "nonfishing" (natural) losses in SNE has occurred. This increase in natural losses does not appear to have occurred in the northern stocks. Circumstantial evidence indicated that natural mortality increased measurably by 1999, when a die-off occurred in LIS (Balcom & Howell 2006). However, as is standard practice, the ASMFC stock assessment model was designed to run with a fixed and low natural mortality value to accommodate the American lobster's long life span, estimated at 20 y. In an effort to quantify what appeared to be biphasic mortality, the ASMFC assessment model was run with the fixed low mortality of 0.15 (15% annual loss) for years 1984 to 1997, and various increased mortality values for years 1998 to 2007 to determine which higher value best fit the observed abundance and length frequency data for those years (Atlantic States Marine Fisheries Commission 2010b). Compared with the "base case" run, which assumed that mortality equaled 0.15 during the entire time period, model runs in which mortality in later years was 1.9 times the base case (i.e., 0.285) had the best fit to the observed stockwide abundance and length data, and exhibited the lowest total unweighted negative log likelihood (Fig. 7). Therefore, subsequent analyses were completed using annual natural mortality rates of 28% after 1997.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
The causes of increases in natural losses are unclear. Any variable that increased, or decreased, about 2-fold starting in the mid 1990s and continued for 15-20 y correlates statistically with the trend in mortality, and therefore can be considered a potential cause of the stock's decline. The problem becomes separating true causal mechanisms from their secondary effects and from purely circumstantial correlations. Both underlying causes and their proximal effects will, in many cases, show similar trends.
THE ROLE OF EPIZOOTIC SHELL DISEASE (ESD)
Laboratory evidence points to an increase in mortality resulting from ESD. Stevens (2009) held 55 shell-diseased lobsters in an ambient flow-through system for 1 y and monitored the progress of each diseased animal. Although this study was limited in scope and included no control (healthy) animals, by year's end, 15% (8 of 55) of the lobsters had died of general unknown causes and an additional 24% (13 of 55) died of complications attributable to shell disease. Interestingly, another 16% (9 of 55) died when exposed to elevated water temperature.
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
The SNE region has experienced an increase in bottom water temperature that is more pronounced than in northern regions (Fig. 8). Temperature elevations of the same magnitude in laboratory settings have affected recruitment by interrupting the normal larval release cycle (Tlusty et al. 2008). In LIS, it appears that the increasing duration of temperature above the lobster stress threshold of approximately 20[degrees]C (Dove et al. 2005) may be most important, rather than an increase in annual maximum or mean temperature. Supporting evidence of a "threshold effect" comes from the long-term record of lobster catches in research traps in eastern LIS off Millstone Power Station (Dominion Nuclear Connecticut 2011). The prevalence of lobsters with signs of ESD correlate significantly with the increased number of days each year with mean temperature above the stress threshold of 20[degrees]C (Fig. 9). Temperature data taken in Buzzards Bay and Woods Hole, MA, show a similar increase in the number of days with mean temperature above 20[degrees]C (Atlantic States Marine Fisheries Commission 2010a), and a similar correlation between disease prevalence and water temperature has been reported for Massachusetts waters (Glenn & Pugh 2006). ESD may therefore be a symptom of weaker animals succumbing to what is actually populationwide temperature stress. Physiological stress, from high temperature and other factors, fits the "host susceptibility model" outlined by Tlusty et al. (2007), in which predisposed stressed animals succumb to an opportunistic disease. These losses would accumulate over successive age groups and could account for the doubling of nonfishing mortality in legal (harvestable)-size lobsters. It should be noted that this relationship between high water temperature and disease prevalence only pertains to lobsters from southern Massachusetts and eastern LIS. ESD prevalence is very low or absent in more southern regions (i.e., western LIS and New Jersey), even though water temperatures are often higher.
It is also likely that many more lobsters showing signs of ESD experience chronic sublethal effects than actual mortality. Length measurements taken from shell-diseased and nondiseased tagged lobsters that were repeatedly recaptured in the Millstone program show that diseased lobsters grew more slowly than nondiseased lobsters in the same size range (Fig. 10) (first reported by Landers in Castro et al. (2006); Dominion Nuclear Connecticut 2011). This difference was greater in females than in males. Similar results were reported by Stevens (2009).
[FIGURE 10 OMITTED]
In summary, the increased mortality outlined here undermines the viability of the SNE stock and the traditional fishery it has supported. To capture these nonharvest effects, ongoing assessment of current and future stock status requires that the population model for the SNE stock be run in segments aligned to predisease/stress and postdisease/stress time periods so that harvest effects can be evaluated in the absence and presence of substantial nonfishing mortality. Sublethal declines in production by egg-bearing females, the most susceptible demographic, are important considerations in modeling the poststress time period. This selectivity in the disease process may play a critical role in suppressing stock rebuilding under traditional management. A modeling approach that incorporates the effects of disease in concert with fishing was developed for crustaceans infected with a nemertean parasite (Kuris & Lafferty 1992), but has not yet been fully developed for ESD and other newly described diseases (Shields et al. 2012) in the SNE lobster stock. Incorporating into the assessment process those factors that (1) stress the population and (2) result in loss of production and additional nonharvest mortality will enable managers to assess more completely not only the impacts of this disease but also to determine whether a sustainable harvest is possible for the SNE lobster stock.
Atlantic States Marine Fisheries Commission. 2009. American lobster stock assessment report for peer review. Stock assessment report 09-01. http://www.asmfc. org/speciesdocuments/lobster.
Atlantic States Marine Fisheries Commission. 2010a. Recruitment failure in the southern New England lobster stock. Technical report to the Lobster Management Board, April 2010. http://www.asmfc.org/speciesdocuments/lobster.
Atlantic States Marine Fisheries Commission. 2010b. Southern New England University of Maine model natural mortality (M) profile. Lobster Technical Committee memo to the Lobster Management Board, June 2010.
Balcom, N. & P. Howell. 2006. Responding to a resource disaster: American lobsters in Long Island Sound, 1999-2004. Connecticut Sea Grant report CTSG-06-02. Connecticut Sea Grant, Groton, CT. 22 pp.
Castro, K, J. R. Factor, T. Angell & D. Landers. 2006. The conceptual approach to lobster shell disease revisited. J. Crustacean Biol. 26(4):646-660.
Dominion Nuclear Connecticut. 2011. Lobster studies. In: Monitoring the marine environment of Long Island Sound at Millstone Power Station, Waterford, Connecticut, Annual report 2010 prepared by the staff of the Millstone Environmental Laboratory. Dominion Resource Services, Inc., Waterford, CT. pp. 167-202.
Dove, A., B. Allam, J. Powers & M. Sokolowski. 2005. A prolonged thermal stress experiment on the American lobster Homarus americanus. J. Shellfish Res. 24:761-765.
Giannini, C. & P. Howell. 2011. Connecticut lobster (Homarus americanus) population studies. CT DEP Marine Fisheries, Old Lyme, CT. MOAA/NMFS Interjurisdictional Grant No. NAo5NMF4071033, Project No. 3-IJ-168, 34 pp.
Gibson, M. & T. Angell. 2006. Estimating the reduction in fishing mortality rate on area 2 lobster associated with the North Cape V-notching program. Rhode Island Division ofFish and Wildlife. Atlantic States Marine Fisheries Commission Lobster Technical Committee Report to the Lobster Management Board. Report to the Atlantic States Marine Fisheries Commission.
Glenn, R. & T. Pugh. 2006. Epizootic shell disease in American lobster (Homarus americanus) in Massachusetts coastal waters: interactions of temperature, maturity, and intermolt duration. J. Crustac. Biol. 26:639-645.
Kuris, A. M. & K. D. Lafferty. 1992. Modelling crustacean fisheries: effects of parasites on management strategies. Can. J. Fish. Aquat. Sci. 49:327-336.
Shields, J., K. Wheeler & J. Moss. 2012. Histological assessment of the lobsters (Homarus americanus) in the "100 lobsters" project. J. Shellfish Res. 31:439-447.
Stevens, B. 2009. Effects of epizootic shell disease in American lobster Homarus americanus determined using a quantitative disease index. Dis. Aquat. Organ. 88:25-34.
Tlusty, M., A. Metzler, E. Malkin, J. Goldstein & M. Koneval. 2008. Microecological impacts of global warming on crustaceans: temperature induced shifts in the release of larvae from American lobster, Homarus americanus, females. J. Shellfish Res. 27:443-448.
Tlusty, M., R. Smolowitz, H. Halvorson & S. DeVito. 2007. Host susceptibility hypothesis for shell disease in American lobsters. J. Aquat. Anim. Health 19:215-225.
PENELOPE HOWELL * Marine Fisheries Division, Connecticut Department of Energy and Environmental Protection, Old Lyme, CT, 06371
* Corresponding author. E-mail: Penny.Howell@ct.gov
|Gale Copyright:||Copyright 2012 Gale, Cengage Learning. All rights reserved.|