According to the Atlantic Sea Scallop Fishery Management Plan, all scallops in the US EEZ belong to a single stock. However, the U.S. sea scallop stock was assessed in two separate components: the Mid-Atlantic Bight, and Georges Bank, and then combined (NEFSC, 2007). There are several reasons for this; the most prominent one being there was an observed difference in recruitment patterns between the two areas.
The 2007 stock assessment incorporated data from many different sources, rather than merely NEFSC scallop dredge survey data. The primary assessment model used is known as CASA, which is size-structured and can directly treat the growth-increment probabilities, compared with other methods that rely on direct estimates of size at age. This model was improved for the most recent stock assessment, making it more precise, with reduced model-specification bias. This improvement has enabled the setting of new Biological Reference Points (BRPs).
Last updated on 19 January 2011
According to the Atlantic Sea Scallop Fishery Management Plan, all scallops in the US EEZ belong to a single stock. However, the U.S. sea scallop stock was assessed in two separate components: the Mid-Atlantic Bight, and Georges Bank, and then combined (NEFSC, 2007). The latest stock assessment published in September 2007 incorporates improved data compilation and interpretation since the previous stock assessment in 2004, with new growth modeling and verification, a revised shell height/meat weight relationship, adjusted shucking capacity, and a re-estimation of selectivity in scallop dredges (Maguire, 2007). For the first time, the 2007 stock assessment incorporated data from many different sources, rather than merely NEFSC scallop dredge survey data. The primary assessment model used is known as CASA, which is size-structured and can directly treat the growth-increment probabilities, compared with other methods that rely on direct estimates of size at age. This model was improved for the most recent stock assessment, making it more precise, with reduced model-specification bias. This improvement, along with new scallop growth information, as well as recent information on increased gear selectivity towards larger scallops and area-based management (rotational closed areas) restrictions, has enabled the setting of new Biological Reference Points (BRP). Previous BRPs were calculated using recruitment estimates based only on the NEFSC dredge survey data and older information available on life history and selectivity. Both the previous and new biological reference points are based on yield-per-recruit, rather than stock size and recruitment estimates. This means the reference fishing mortality rate (F) used is FMAX rather than FMSY, and in order to produce optimum yield-per-recruit, the target F for the sea scallop fishery should be set between 0.20 and 0.30 (EPA, 2006). Since 1998, F has ranged from 0.18 to 0.34 (Figure 1), while FMAX in 2007 is 0.24 for Georges Bank (GB) and 0.25 for the Mid-Atlantic sea scallops (Hart, 2006b).
Figure 1. Scallop landings and fishing mortality rate, 1982-2005.
Data taken from (Hart, 2006b) Status of Fishery Resources off of the Northeastern U.S., NEFSC – Resource Evaluation and Assessment Division, December 2006 (Atlantic Sea scallop).
Keeping F under FMAX will prevent growth overfishing, but will not necessarily protect against recruitment overfishing, meaning the adult population could still be removed to an extent that compromises successful replenishment of stocks. Recent gear changes resulting in selection tendencies towards larger scallops appear to have helped improve yields, and decrease discards, which, together with closed areas, may help protect against recruitment overfishing. This may alleviate some of the concerns associated with using yield-per-recruit based reference points. Overall, sea scallops as a resource are inherently different from fish populations, as they have a benthic existence, but also form relatively sedentary communities. Their larval dispersal may allow for spawners in closed areas to contribute to an increase in overall stock, inside and outside of closed areas.
In the U.S., scientific advice and management decisions are given and made by the same organization: NOAA Fisheries Service. Advice from independent organizations, such as academic institutions and research organizations is also closely followed when making management decisions. It is suggested that the target F for the sea scallop fishery should be set between 0.20 and 0.30 (EPA, 2006).
The improvement of the primary assessment CASA model enable the setting of new Biological Reference Points (BRPs) which are based on yield-per-recruit, rather than stock size and recruitment estimates, meaning that reference fishing mortality rate (F) used is FMAX, or, Fthreshold, rather than FMSY.
The biological reference points adopted are as follows:
Pre-2007 BRPs FMAX = 0.24 (FMSY proxy) Ftarget = 0.20 (80% of threshold) Bthreshold = 2.8 kg/tow Btarget = 5.6 kg/tow (BMSY proxy) | 2007 BRPs FMAX = 0.29 (FMSY proxy) Ftarget = 0.20 Bthreshold = 54.3 kmt Btarget = 108.6 kmt (BMSY proxy) |
Last updated on 19 January 2011
In the U.S., scientific advice and management decisions are given and made by the same organization: NOAA Fisheries Service. The decision making process is more complex than it sounds, however. The Northeast Fisheries Science Center (NEFSC), which runs under the auspices of the National Oceanic and Atmospheric Administration (NOAA) Fisheries Service, runs the fisheries independent scallop dredge survey, on which a large portion of the scientific advice is based. Advice from independent organizations, such as academic institutions and research organizations, is also closely followed when making management decisions. Data used in the Atlantic sea scallop stock assessment is contributed by: the NEFSC’s annual federal scallop dredge survey; the School for Marine Science and Technology at University of Massachusetts, Dartmouth (SMAST) video survey for reassessment of dredge survey selectivity for scallops less than 40 mm in shell height; the NEFSC Fisheries Observer Program; and Vessel Monitoring Systems (VMS) (NEFSC, 2004; Maguire, 2007). Although the scallop industry began to fund their own observer program in 2003, policy issues forced a suspension of the program from 2004-2007, during which time observers were provided and funded by the federal government. This resulted in a lower percentage of observed fishing trips during the suspension. In 2007, the implementation of Amendment 13 to the Atlantic Sea Scallop Fishery Management Plan enabled the reinstatement of the industry funded program.
Last updated on 19 Jan 2011
A new stock assessment for Atlantic sea scallops was published in September 2007. Both the previous and the new reference points are based on yield-per-recruit, rather than stock size and recruitment estimates. This means the reference fishing mortality rate (F) used is FMAX, or, Fthreshold, rather than FMSY. Therefore, assigning a simplified biomass target value is attempted solely as a convenience and/or necessity for management. In the new stock assessment, estimates of F and biomass (B) have made considerable progress since the previous assessment update in 2005. The Mid-Atlantic and Georges Bank (GB) areas are now modeled separately before results are combined (NEFSC, 2007). There are several reasons for this, the most prominent one being there was an observed difference in recruitment patterns between the two areas.
The 2005 F estimate was 0.22, which is below the threshold or FMAX of 0.24 (used as a proxy for FMSY), but above the target F (80% of threshold, or 0.20). The new stock assessment shows an increase in the suggested estimate of FMAX from 0.24 to 0.29 since the previous stock assessment update in 2005. Although F has been above Ftarget (0.20) for most recent years, it remains below FMAX. Because F is different for each area, and there is rotational area management, it is difficult to maintain a rate below the target F. However, the recent increase in FMAX shows that F exceeding Ftarget is not currently a concern.
In previous stock assessments, relative sea scallop abundance was estimated by measuring the weight of scallops in kg/tow from annual scientific surveys. Estimates were based on older life history and selectivity information, and recruitment estimates were based solely on data from the NEFSC scallop dredge survey. These annual measurements serve as a good proxy for total biomass. Target biomass levels for 2006 were 5.6 kg/tow (calculated as BMAX x median recruitment and used as a proxy for BMSY) and the NEFSC dredge survey yielded actual biomass levels of 7.7 kg/tow, indicating biomass levels were well above target levels (NOAA, 2007; NEFSC, 2006b). The biomass threshold, below which the stock would be defined as overfished, is one half the target biomass level.
The new draft stock assessment estimates biomass in meat weights of the whole stock (just the weight of the scallop meats, i.e., minus the viscera and shell weights). These estimates are based on new life history and selectivity information, and recruitment estimates for the stock based on a new model, which allows for growth increments, rather than depending on size-at-age data. They also depend on well established shell height/meat weight relationships for each area.
Table 3. Biological Reference Points
According to the new BRPs, Atlantic sea scallops are not overfished and overfishing is not occurring (NEFSC, 2007).
F2006 = 0.23
FMAX= 0.29
B2006 = 166,200 tonnes meats
BThreshold = 54,300 tonnes meats
Historically, the Atlantic sea scallop fishery has experienced dramatic cycles of biomass oscillations (usually more than ten years per cycle), categorizing it as a “boom and bust” fishery.
In the period between 1998 and 2001, landings increased 264% from 5,879 tonnes to 21,404 tonnes (Baskaran and Anderson, 2005). The dramatic increase was due partially to the opening of Closed Area II in 1999. Exploitable biomass also increased dramatically from 55,014 tonnes in 1998 to 116,350 tonnes in 2001. Landings and biomass peaked in 2004 at the high point of the current cycle. Even during the downwards portion of the cycle, projections account for these cyclical decreases, which helps to manage the resource effectively, and allow for the stock to remain at a healthy status.
Last updated on 19 January 2011
Last updated on 19 Jan 2011
Since 1998, F has ranged from 0.8 to 0.34, while FMAX in 2007 was 0.25 for the Mid-Atlantic Sea scallops (Hart, 2006b). The 2005 F estimate was 0.22, which is below the threshold or FMAX of 0.24 (used as a proxy for FMSY), but above the target F (80% of threshold, or 0.20). Since the previous stock assessment update in 2005, the suggested estimate of FMAX increased from 0.25 to 0.29. Although F has been above Ftarget (0.20) for most recent years, it remains below FMAX.
Historically, the Atlantic sea scallop fishery has experienced dramatic cycles of biomass oscillations (usually more than ten years per cycle), categorizing it as a “boom and bust” fishery. In 1994, when landings and biomass were at a low point in their cyclical variation (Figures 3, 4), fishery managers closed three large areas on Georges Bank to scallop fishing. Portions of these areas have been reopened to fishing for intermittent periods since 1999. This system of rotational closures has aided an increase in scallop biomass, and may help prevent stock collapse, as well as conserve biodiversity (Hart, 2006a). In the period between 1998 and 2001, landings increased 264% from 5,879 tonnes to 21,404 tonnes (Baskaran and Anderson, 2005). The dramatic increase was due partially to the opening of Closed Area II in 1999. Exploitable biomass also increased dramatically from 55,014 tonnes in 1998 to 116,350 tonnes in 2001. Landings and biomass peaked in 2004 at the high point of the current cycle (Figures 3, 4). Even during the downwards portion of the cycle, projections account for these cyclical decreases, which helps to manage the resource effectively, and allow for the stock to remain at a healthy status.
Figure 3. Biomass in open and closed areas, 1979-2005.
Data taken from (Hart, 2006b) Status of Fishery Resources off of the Northeastern U.S., NEFSC – Resource Evaluation and Assessment Division, December 2006
Source : (Status of Fishery Resources off the Northeastern US).
Figure 4. Total commercial landings of scallops, 1887-2005.
Data taken from (Hart, 2006b) Status of Fishery Resources off of the Northeastern U.S., NEFSC – Resource Evaluation and Assessment Division, December 2006
Source : (Status of Fishery Resources off the Northeastern US).
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American sea scallop - US Atlantic, Mid-Atlantic Bight, US 6A,B,C, United States, Towed dredges