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CONTENTS
Introduction
Life History
Fisheries Dependent Data
Fisheries Independent Data
ASAP Model
Projections
Conclusion
Literature Cited
Appendix 1. ASAP Model Output

Northeast Fisheries Science Center Reference Document 10-15

Bluefish 2010 Stock Assessment Update

Gary R. Shepherd and Julie Nieland
NOAA Fisheries, Northeast Fisheries Science Center, 166 Water Street, Woods Hole MA 02543

Web version posted August 3, 2010

Citation: Shepherd GR, Nieland J. 2010. Bluefish 2010 stock assessment update. US Dept Commer, Northeast Fish Sci Cent Ref Doc. 10-15; 33 p. Available from: National Marine Fisheries Service, 166 Water Street, Woods Hole, MA 02543-1026, or online at http://nefsc.noaa.gov/publications/

Information Quality Act Compliance: In accordance with section 515 of Public Law 106-554, the Northeast Fisheries Science Center completed both technical and policy reviews for this report. These predissemination reviews are on file at the NEFSC Editorial Office.

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Executive Summary

The updated stock assessment was completed by adding catch and indices through 2009 to the previous 2008 assessment. Catch information consisted of commercial landings and length frequencies from Maine to Virginia collected by the Northeast Fisheries Science Center, North Carolina landings and length information collected by NC Division of Marine Fisheries, Florida landings and length information collected by FL Fish and Wildlife Research Institute, and recreational landings and discards from Maine to Florida collected in the NMFS recreational fisheries survey.  The catch data was combined with fisheries independent survey data from the Northeast Fisheries Science Center, DE DNR, NJ DEP, CT DEP, coast-wide recreational catch per angler as well as recruitment indices from the SEAMAP program in the South Atlantic in the forward projecting catch at age model ASAP.  Fishery dependent and independent information was partitioned into ages using a 2009 age-length key developed by Old Dominion University.

The result of the analysis shows that bluefish is not overfished or experiencing overfishing.  Fishing mortality in 2009 was 0.10, below the biological reference point (FMSY) of 0.19. Fishing mortality steadily declined from 0.31 in 1987 to 0.12 in 2002 and has remained steady since 2000 with an average F=0.14. Recent mean biomass estimates peaked in 1982 at 425.0 thousand MT, then declined to 103.8 thousand MT by 1996 before increasing to the 2009 level of 156.0 thousand MT.  Recruitment estimated in the ASAP model has remained relatively constant since 2000 at around 22.5 million age-0 bluefish, with the exception of a relatively large 2006 cohort estimated as 35.2 million fish. The 2009 recruitment estimate was well below average at 8.0 million fish.  There was no significant retrospective bias in the results.  A projection of the results through 2012, under five different fishing scenarios, suggest that biomass will decline at fishing above status quo F due to a very poor incoming year class. However, abundance increases with F less than 0.18.   Changes in NMFS survey, limited age information and discard size data all contribute to the uncertainty in the assessment.

Introduction

The Atlantic coast stock of bluefish (Pomatomus saltatrix), distributed from Maine through eastern Florida, is jointly managed by the Atlantic States Marine Fisheries Commission (ASMFC) and the Mid-Atlantic Fishery Management Council (MAFMC). A total annual quota is established and allocations given to commercial and recreational fisheries. The management plan requires a distribution of 80% to recreational and 20% to commercial, with provisions to shift unused recreational quota to commercial fisheries.

  A bluefish stock assessment was presented for peer-review at the Northeast Fisheries Science Center Stock Assessment Review Committee meeting (NEFSC SARC 41).  The reviewers accepted the assessment for use in management decisions although there were some reservations about the modeling approach.  Since the review, the bluefish stock assessment sub-committee (SASC) has produced annual updates while maintaining the basic model settings from the approved assessment. The current assessment is a continuation of the model update with the addition of 2009 catch at age and indices at age information.

Life History

Bluefish, Pomatomus saltatrix, is a coastal, pelagic species found in temperate and tropical marine waters throughout the world (Goodbred and Graves 1996; Juanes et al. 1996). Bluefish spawn in offshore waters (Kendall and Walford 1979; Kendall and Naplin 1981). Larvae develop into juveniles in continental shelf waters and eventually move to estuarine and nearshore shelf habitats (Marks and Conover 1993; Hare and Cowen 1994; Able and Fahay 1998; Able et al. 2003). Bluefish are highly migratory along the U.S. Atlantic coast and seasonally move between the U.S. south Atlantic and Middle-Atlantic, traveling as far north as Maine (Shepherd et al., 2006).  

Several studies show bluefish to be a moderately long-lived fish with a maximum age of 14 years (Hamer 1959; Lassiter 1962; Richards 1976; Barger 1990; Chiarella and Conover 1990; Terceiro and Ross 1993; Austin et al. 1999; Salerno et al. 2001; Sipe and Chittenden 2002). Bluefish up to 88 centimeter (cm) fork length (FL) have been aged (Chiarella and Conover 1990; Salerno et al. 2001), although Terceiro and Ross (1993) noted considerable variation in mean bluefish size-at-age. Scale ages have been used to estimate von Bertalanffy growth parameters (Lassiter 1962; Barger 1990; Terceiro and Ross 1993; Salerno et al. 2001). The values for Lfrom these studies (87-128 cm FL) match closely to the largest individuals in catch data and growth rates do not differ between sexes (Hamer 1959; Salerno et al. 2001).

Bluefish grow nearly one-third of their maximum length in their first year (Richards 1976, Wilk 1977). Variation in growth rates or sizes-at-age among young bluefish is evident from the appearance of intra-annual cohorts. Lassiter (1962) identified a spring-spawned cohort and a summer-spawned cohort from the bimodal appearance of size at Annulus I for fish aged from North Carolina and the seasonal cohorts can differ in age by two to three months. Summer-spawned larvae and juveniles grow faster than spring-spawned larvae and juveniles (McBride and Conover 1991) although size differences at annual age diminish greatly after three to four years (Lassiter 1962).

Spawning occurs offshore in the western North Atlantic Ocean, from approximately Massachusetts to Florida (Norcross et al. 1974; Kendall and Walford 1979; Kendall and Naplin 1981; Collins and Stender 1987). Bluefish are characterized as multiple spawners with indeterminate fecundity which spawn continuously during their spring migration (Robillard et al. 2008).  In addition to distinctive spring and summer cohorts, Collins and Stender (1987) identified a fall-spawned cohort, demonstrating the potential of an extended bluefish spawning season.

Bluefish in the western North Atlantic are managed as a single stock (NEFSC 1997; Shepherd and Packer 2006). Genetic data support a unit stock hypothesis (Graves et al. 1992; Goodbred and Graves 1996; Davidson 2002). For management purposes, the ASMFC and MAFMC define the management unit as the portion of the stock occurring along the Atlantic Coast from Maine to the east coast of Florida.

Fisheries Dependent Data

Annual catch information was developed for five components of the commercial fishery. Commercial landings from Maine to Virginia, North Carolina commercial landings, Florida commercial landings, coast-wide recreational landings and coast-wide recreational discards. 

Commercial fisheries from Maine to Virginia were sampled as part of the NEFSC data collection program.  Lengths were sampled from a variety of gears and market categories. Expansion of length data was completed by market category and quarter of the year, with the results merged into half year periods. In 2009 a total of 4,525 measurements from 102 samples were collected across all market categories from total landings of 1,959 mt (62% of all commercial landings; Table 1).  Market category/quarter with inadequate length samples were filled with length information from adjacent quarters within the same market category or from NC samples if necessary.

North Carolina commercial landings were expanded using length samples collected by NC Division of Marine Fisheries.  A total of 7,155 measurements from 73 samples were collected from landings of 1,096 mt (Table 1).  Expansion of landings at length were done by quarter, market category and gear type then combined into half year totals.  Length samples from Florida 2009 commercial landings were also available.  A total of 654 lengths from 22 samples were used to expand commercial landings of 97 mt (Table 1). No landings were reported for South Carolina or Georgia . Total coast-wide commercial landings in 2009 were 3,151 mt, an increase from 2,585 mt in 2008.

Length frequencies from commercial fisheries are characterized by a multi-modal distribution (Figure 1). In 2009 the distribution was strongly bimodal with one peak at 39 cm and a second around 65 cm. There were few fish below 25 cm. In comparison, the 2006 and 2008 distribution included a third mode around 55 cm. The 2007 distribution was bimodal, similar to 2009 except the first mode peaked around 25 cm.

Recreational landings are sampled for length as part of the MRFSS program.  The 2009 recreational landings were 6,161 mt, a decrease from 8,573 mt in 2008 (Table 2, Figure 2). The MRFSS 2009 length samples (N=6,066) were used to expand recreational landings per half year.  Recreational discards in 2009 were estimated at 6,403 mt, however after adjusting for a 15% mortality rate, the resulting discard loss was 960 mt. A recent publication (Fabrizio et al 2008) shows that mortality may be higher and the 15% should be reevaluated in the next benchmark assessment. Length sampling for bluefish discards was limited.  MRFSS at-sea sampling of recreational party boats provided lengths of 765 discarded bluefish.  Total combined (commercial and recreational) length frequencies are presented in Figure 3.

Age data (n=380) was provided by Virginia Marine Resources Commission and Old Dominion University ageing lab.  Since the age key developed from the VA samples was the only 2009 age information available, it was applied to both fishery dependent and independent length data.  Age data was provided by cm, fork length by half year. In the previous year the fish were measured to total length, inches while the length frequencies were measured in fork length to the nearest cm. Consequently, previous length frequencies were converted to TL, inches using the following equation: 

TL(in) = 0.245(FL(cm)) +0.440

The resulting catch at age through 2009 is presented in Table 3. As in previous bluefish assessments the ages are summarized in a plus category for ages 6 and above to reduce the effect of aging error. Adjustments were made in the 2004-2006 catch at age data to reflect new MRFSS catch estimates.

Fisheries Independent Data

Survey indices as used in the previous bluefish assessment were updated for 2009. These indices include SEAMAP juvenile (age 0) indices, Northeast Fisheries Science Center (NEFSC) bottom trawl survey indices for ages 0 to 6+, NJ bottom trawl survey indices of ages 0 to 2, CT bottom trawl survey indices for ages 0 to 6+, DE bottom trawl survey indices for ages 0 to 2 and Marine Recreational Fisheries Statistics Survey (MRFSS) recreational catch per angler trip (CPA) for ages 0 to 6+ (adjusted using a general linear model with negative binomial transformation).  The CT survey in 2008 was not conducted during the month of September, therefore the 2008 index was treated as missing data. The NEFSC survey in 2009 was modified by the replacement of the FV Albatross IV with the FSV Henry B. Bigelow. The consequence of the replacement was a change in the areas surveyed and the efficiency of the survey due to a change in net size and towing speed (as well as other intangibles associated with a different vessel).  Beginning in 2009 only the outer third of the inshore strata set was sampled by the Bigelow. In addition, a conversion coefficient of 1.16 was used to convert Bigelow mean number per tow into equivalent Albatross units (Miller et al., 2010).

Among these survey indices, there were no consistent trends in total abundance. The total NEFSC index (ln re-transformed stratified mean number per tow) declined to 7.0 in 2009 from 12.8 in 2008 (Table 4). The series arithmetic average index equaled 27.3 (geometric mean of 14.26). The 2009 Delaware survey index of ages 0 to 2 was 0.342 fish per tow, below the time series average (0.531 per tow; Table 5). New Jersey trawl survey indices of ages 0 to 2 for 2009 (3.2 fish/tow) was also below the time series average of 6.6 per tow (Table 5).  Indices of bluefish abundance in Long Island Sound from the CT DEP survey for ages 0 to 6+ in 2009 (32.86 per tow) were about average for the series (33.42 per tow; Table 6).  In contrast, recreational catch per angler trip showed the highest annual increase in the time series, increasing from 0.37 fish per angler trip in 2008 to 0.83 in 2009 (Table 7). The increase was most prevalent at age 1 but was above average at all ages except age 0.

Standardized recruitment indices (age 0) were developed using Z scores to compare the relative 2009 indices to time series averages. Indices from NEFSC, DE, NJ, and the Recreational CPA were all below average, CT was slightly above average whereas the SEAMAP index was well above average (Table 8, Figure 4).

ASAP Model

The ASAP model (version 2.0.19) was run with the previous 1982-2008 input file updated for 2009 total catch, catch at age and indices at age.  The fishery was modeled as a single fleet with selectivity fixed as a bimodal pattern with full recruitment at age 1 (coded age 2).  Model weighting factors remained the same as previous assessments with the model heavily weighted towards the fishery total catch rather than survey indices.  Weights at age remained constant since 2005. Natural mortality was fixed at 0.2 and maturity at age was held constant with full maturity at age 3.  

The results of the ASAP model showed a decrease in total abundance since 2007, declining from 97.9 million to 77.7 million (Table 9, Figure 5). The decline is primarily the result of a poor 2009 year class. Recruitment has remained relatively constant since 2000 around 22.5 million age-0 bluefish, with the exception of a large 2006 cohort estimated as 35.2 million fish. The 2009 recruitment estimate was well below average at 8.0 million fish compared to the series average of 23.1 million (Figure 6).  However among other age groups, the 6 plus category was the largest since 1991. Total biomass in 2009 was the highest since 1989 and equaled 155,991 mt. (Table 10) The corresponding spawning stock biomass (SSB) at 126,200 mt was also the highest since 1989 (Figure 7). The 2009 estimate of total biomass is above the biomass necessary to sustain maximum yield (BMSY) of 147,052 mt (Figure 7).

Fishing mortality estimates in ASAP are based on a separability assumption with F at age the product of FMULT and selectivity. Full selectivity is fixed at age 1.  The 2009 FMULT value equals 0.10 (Figure 5). Fishing mortality steadily declined from 0.31 in 1987 to 0.12 in 2002 and has remained steady since 2000 with an average F=0.14. The 2009 estimate of fishing mortality (0.10) is above the biological reference point for FMSY at 0.19.

Retrospective bias for the final model was examined for F, total abundance, recruitment (age 0) and total biomass. The analysis shows little evidence of significant bias in the estimates (Figure 8). The variation in the final model results for F and SSB was determined using a Monte Carlo Markov chain with 2500 iterations. The MCMC results of variation around F ranged from 0.096 to 0.115, with the 80% CI between 0.099 and 0.109. Estimates for SSB ranged from 111,800 to 133,500 mt, with an 80% CI between 117,200 mt and 128,100 mt. (Figure 9).

Projections

Bluefish abundance and biomass through 2012 was examined for a range of fishing scenarios with a stochastic projection in AGEPRO software.  The weight at age in 2010-2012 was assumed equal to 2009. Recruitment was derived from a random draw of 28 empirical estimates of age 0 abundance since 1982 and population size was drawn from the output of the MCMC run.  Fishing mortality for 2010 was assumed equal to 2009.  Five projection scenarios were examined: F status quo (0.10), F equal to 75% of FMSY (0.14), Ftarget (0.17) which equals 90% of FMSY as defined in FMP, F0.1 (0.18) from the yield per recruit, and FMSY (0.19)  

Results of the projections show a decrease in mean biomass and SSB for each scenario other than status quo F (F=0.10) (Table 11). However, abundance would continue to increase for F less than 0.18, and in all 5 cases the yield through 2012 would increase above the 2010 yield of 9,563 mt. Under status quo F, projected 2012 yield would increase to 10,821 mt, which includes commercial and recreational landings as well as recreational discards losses.

Conclusion

The conclusion of the updated assessment is that the Atlantic coast bluefish stock continues to remain above the biological reference points (FMSY and BMSY) and is not considered overfished or experience overfishing.  The estimates of the model show little variation or significant retrospective patterns. The lack of variation is due in part to the fixed parameters for selectivity. Nevertheless, uncertainty remains in several aspects of the assessment input data. Age data continues to be limited to one age key from a limited set of samples. The assumption that this age information is applicable to all areas remains untested. Length samples from recreational discards are limited and contribute to the uncertainty as does the lack of commercial discard estimates. Changes in the NEFSC inshore survey series, from both vessel changes and sample area adjustments, significantly alter indices. Strata inshore of 15 fathoms are currently sampled as part of the NEMAP survey, but the time series is not yet adequate to provide a tuning index.

The highly migratory nature of bluefish populations and the recruitment dynamics of the species create a unique modeling situation. Migration creates seasonal fisheries with unique selectivity patterns resulting in a bimodal partial recruitment pattern. This pattern has been identified in previous assessments as a source of uncertainty in the results and has been held constant in the model. The migratory pattern in bluefish also results in several recruitment events. A spring cohort, originating south of Cape Hatteras, NC during spring migrations, and a summer cohort originating in the offshore Mid-Atlantic Bight result in a bimodal age-0 size distribution. It has been hypothesized that the success of the spring cohort controls the abundance of adult bluefish. Future assessments should include any additional information that could index seasonal abundance of incoming recruitment.

Literature Cited

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