This article provides an assessment of the status of yellowtail
snapper in the southeast United States through 2003.
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A stock assessment of
yellowtail snapper,
Ocyurus chrysurus, in the Southeast United States
Robert G. Muller, Michael D. Murphy, Janaka de Silva, and
Luiz R. Barbieri
Florida Fish and Wildlife Conservation Commission
Florida Marine Research Institute
St. Petersburg, FL
EXECUTIVE
SUMMARY
(revised for Web site)
The status of yellowtail snapper was assessed through the
National Marine Fisheries Service's SEDAR process with the Florida
Fish and Wildlife Conservation Commission (FWC) taking the lead.
The SEDAR process consists of three workshops. The data workshop
was held 3-4 March 2003 at FWC's Florida Marine Research Institute
in St.Petersburg, and the Stock Assessment Workshop was held 9-13
June 2003 at the same venue. The Peer-Review Workshop was held
28-31 July 2003 in Tampa, Florida.
The following is a summary of the biology, fishery, and
assessment of yellowtail snapper with comments about important
discussions and conclusions made by the Stock Assessment Workshop
Panel and the Peer-Review Panel.
Yellowtail snapper, Ocyurus chrysurus, is a reef fish species
that occurs from North Carolina to southern Brazil and is abundant
in south Florida. Adult yellowtail snapper typically inhabit sandy
areas near offshore reefs at depths of 10 m-70 m (32-230 feet).
Yellowtail snapper eat fish, shrimp, and crabs near the bottom but
also feed in the water column.
The spawning season in south Florida is during spring and summer
with a peak during May-July. Females reach the 50% maturity ogive
at 209 mm TL at an average age of 1.7 years. Yellowtail snapper
grow quickly initially, but size is a poor indicator of age because
of the extensive overlap in ages for a given sized fish. The Data
Workshop Panel recommended not pursuing sex-specific differences in
growth because, based on the analysis, the available data did not
show any obvious differences in size at age between sexes. There
were detailed discussions about the potential difficulties in using
agestructured assessment approaches when so much variability in
length at age was observed. The Stock Assessment Panel finally
agreed on the assessment approach after looking at catch-at-age
data generated using direct aging of the catch and various pooling
strategies for the development of age-length keys. Based on the
maximum age of sampled yellowtail snapper (17 years old, confirmed
since the Data Workshop) and the established nature of the fishery,
the panel recommended using a lower natural mortality rate than
suggested at the Data Workshop. A baseline instantaneous rate of
0.2 yr-1 was used with additional runs also at 0.15 yr-1 and 0.25
yr-1. After discussion, the Peer-Review Panel found no reason to
change either the baseline rate or the range.
The commercial fishery for yellowtail snapper occurs throughout
the tropical, western Atlantic. Average landings from the Caribbean
for 1997-2000 have been 3,458 metric tons (mt), and of that total,
the United States landings have averaged 747 mt with Puerto Rico
and the U.S. Virgin Islands accounting for another 220 mt. The
fishery has occurred in the Florida Keys for over a century and
mostly uses hook-and-line gear, especially after entangling gear
was prohibited in 1990, five years before Florida's Constitutional
amendment banning the use of entangling gear from state waters.
For this stock assessment, data from the yellowtail snapper
fisheries were divided into two regions: the Atlantic region, which
primarily is from Palm Beach County south through Miami-Dade
County, and the Keys, which is Monroe County and all counties to
the west. The Stock Assessment and Peer-Review panels concurred
with the approach to estimate catch-at-age for the MRFSS
recreational, headboat, and commercial sectors separately for the
Atlantic (Miami-Dade County north) and Keys regions (Monroe County
north). Although there are commercial landings data from earlier
years, recreational data are only available since 1981; therefore,
we have confined the analyses to the years 1981-2001.
Total landings during these years increased from 1,000 mt in
1981 to 1,648 mt in 1993 and then decreased to 802 mt in 2001.
Despite Data Workshop panel recommendations that sensitivity
analysis include temporal increases in unreported commercial catch,
the Stock Assessment Panel did not recommend including this as a
sensitivity analysis due to the lack of empirical evidence for
changes in reporting. Effort followed a trend similar to that of
total landings, increasing to a peak and then decreasing. The
number of commercial fishers has decreased from a peak of 8,343
Saltwater Products license (SPL)holders in 1989 to 2,659 SPLs in
2001. Recreational trips declined from 2.3 million trips in 1988 in
the Atlantic regions to 1.7 million trips in 2001 and from 1.2
million trips in 1993 in the Keys to 0.4 million trips in 2001.
Similarly, the headboat effort was highest in 1981 with 155,000
angler-days in the Atlantic region and generally declined to 63,000
angler-days in 2001 and from 82,000 angler-days in 1989 in the Keys
to 45,000 anglerdays in 2001.
The Stock Assessment Panel discussed the estimation of
commercial discard rate and discard mortality and agreed to use the
preliminary discard data from commercial logbooks instead of the
10% discard mortality rate suggested during the Data Workshop. The
Peer-Review Panel noted that the paucity of discard data was
unsatisfactory and fishers on the panel indicated that these rates
were too high. In the assessment runs, we increased the landings to
account for discards. Based on a single year's reef fish logbook
data in 2001-2002, commercial discards of yellowtail snapper
averaged 16% of the landings, and approximately 28% of those
discarded were dead. Recreational discards are estimated directly
as Type B2 numbers of fish. With the absence of headboat discard
information, the Stock Assessment Panel concluded, after much
discussion and examination of the age distribution of the
fishery-dependent and fishery-independent samples by region for
1999-2001, that the proportion of fish that would have been
discarded by headboat could be assumed to equal the fraction of the
catch of the fishery-independent hook-and-line data that was
smaller than the legal size limit (305 mm TL)(37% in the Atlantic
region and 27% in the Keys region). The panel discussed the 30%
discard mortality rate used with the recreational and headboat
fisheries and found insufficient evidence to suggest changes to
these rates.
Commercial landings in weight were converted to landings in
number based on biostatistical sampling of the landings that
measured lengths from landings with different gear. Biostatistical
samplers visit fish houses, interview fishers, measure fish, and
collect hard parts for age determinations. Landings are estimated
directly in numbers in the recreational fisheries. Ages were
assigned to the lengths based on region, fishery, gear, and
year.
The Stock Assessment Panel noted differences in age composition
between the Atlantic and Keys for all fishery-dependent and
fishery-independent data. This was most evident for maximum age: 7
years in the Atlantic and 17 years in the Keys. The age-length data
were sufficient from 1997 onward to derive year-specific age-length
keys for both regions. The panel noted that the composite
age-length keys could act to obscure yearclass strength
information. For earlier years when sample sizes were insufficient,
the Stock Assessment Panel recommended combining data from the same
region and year but from different gear. When data from alternative
gear were not available, the second choice for substitution was to
use data from the same region and from different gear in different
years. A composite was formed for the years 1980-1986 and
1987-1996. Finally, the panel investigated using age data from 1994
through 2001 to directly age the catch. However, the abrupt change
in the younger ages from the composite age-length keys to direct
aging led the panel to recommend not using the direct aging
method.
We used tuning indices to improve the statistical population
models. The two fisheryindependent indices were based on visual
surveys conducted by the National Marine Fisheries Service and the
University of Miami. These indices were the number of fish less
than 197 mm (7¾ inches) per 177 m² that was used for age-1 fish and
the number of fish greater than 197 mm per 177 m² that was used for
fish age-2 and older. The panel discussed the change in the number
of strata used to develop these indices. However, a subsequent
conversation with the analyst that developed the indices confirmed
that these indices were the most comparable and that the increase
in the number of strata was to account for protected areas in the
Keys and the partitioning of patch reefs to afford finer
resolution. The panel rejected the use of the third
fishery-independent index, REEF visual survey. The coarseness of
the classification of abundance, i.e. 0, 1-10, 11-100, >100
individuals, was considered to be too great to use the REEF index
as a quantitative index for yellowtail snapper abundance.
In addition to the fishery-independent indices, we originally
developed five fishery-dependent indices that were standardized
with generalized linear models:
- Commercial kilograms per trip with combined gear
(1985-2001)
- Commercial kilograms per hook-and-line trip from trip tickets
(1992-2001)
- Commercial kilograms per hook-and-line trip from Reef Fish
Permit logbooks (1993-2001)
- MRFSS recreational total number of fish caught per trip
(1981-2001)
- Headboat number of fish landed per trip that was divided into
two time periods, 1981-1991 and 1992-2001 because of the aggregate
bag limit
After much discussion, the panel agreed that the original CPUE
indices for headboat and commercial sectors derived by the analysts
before the assessment workshop were valid indices. However, these
indices were derived under the philosophy of including many reef
trips, only coarsely filtered for yellowtail snapper trips. The
panel also felt that another set of valid CPUE indices should be
derived based on anglers that were targeting yellowtail snapper. We
developed two additional indices: the kilograms per trip, from
commercial hook-and-line trips by 107 Reef Fish Permit holders that
landed at least 500 kilograms of yellowtail snapper in five of the
most recent seven years, and headboat indices, from seven vessels
that landed at least 100 yellowtail snapper per year. The
Peer-Review Panel pointed out that including interaction terms with
year in the indices might not reflect underlying population changes
and recommended calculating the indices with just main effects.
They also requested an analysis without the commercial index
because they thought that perhaps the increase in that index was
due to increased efficiency instead of a population increase. The
run without the commercial index produced the same trends as
before.
Finally, the Stock Assessment Panel noted that the flat CPUE
indices with declining landings implied declining effort.
Subsequent analyses requested by the panel confirmed declining
annual number of angler-days for the headboat sector and declining
overall number of trips in the MRFSS recreational and the
commercial sectors.
We used two types of models to assess the condition of
yellowtail snapper: surplus production and age-structured,
statistical models. However, the two surplus production models,
ASPIC, a non-equilibrium model, and ASP, an age-structured model,
were not stable, and most likely, the instability was due to lack
of contrast in the tuning indices or catch rates. The Stock
Assessment Panel noted that the generally flat or monotonic CPUE
indices could create parameter-estimation-convergence issues with
surplus production models.
Both the Stock Assessment and Peer-Review panels agreed with the
Data Workshop recommendation that age-structured assessment
approaches were appropriate for yellowtail snapper. Year-specific
aging information was available for 1994-2001. Age-structured
approaches could make use of all available data, lending more
confidence in the analyses' predictions of the current status of
the stock.
We used two age-structured, statistical models. The first was
Integrated Catch-at-Age which used the combined catch-at-age from
the three fisheries and tuning indices to estimate the population
sizes by age in the most recent year, fishing mortality rates on
the earliest fully recruited age of fish, selectivity patterns by
age, and catchability coefficients for the tuning indices (76
parameters in this configuration). In the base case run, the full
fishing mortality rate in 2001 was 0.21 per year and the spawning
biomass in 2001 was 4,943 mt.
The numbers of age-1 fish and the spawning biomass a year
earlier were used to estimate the biomass-based management
benchmarks given a steepness of 0.8 and alternatives of 0.7 and
0.9. The steepness is merely the proportion of the recruitment at a
spawning biomass of 20% of the virgin biomass to the recruitment at
the virgin biomass. With the Stock Assessment Panel recommendation
of using a steepness value of 0.8, the maximum sustainable yield
(MSY) was 941 mt; the F2001/FMSY ratio was 0.62, and the
SSB2001/SSBMSY ratio was 1.35, indicating that the stock was not
undergoing overfishing and not overfished. The ratios were 0.57 and
1.43 when the analyses were rerun using indices calculated without
the interaction terms.
The second age-structured model allows simultaneous estimation
of separate fishing mortality rates for the three fisheries. This
fishery-specific model estimated the population sizes in the first
year (1981), recruitment from a stock-recruit
relationship,selectivities by fishery and by two periods
corresponding to before and after the 12-inch (305 mm) size limit
was implemented in 1983, and catchability coefficients for the
tuning indices. This model estimated the sum of the fishing
mortality rates on fully recruited fish in 2001 at 0.24 per year
and a spawning biomass of 5,200 mt, which is similar to the 0.21
per year and 4,900 mt estimated by ICA. The fishery-specific model
estimated a higher MSY of 1,366 mt but only a slightly higher FMSY
(0.36 per year as compared to 0.33 per year from ICA). The
biomass-based benchmarks were F2001/FMSY = 0.65 and SSB2001/SSBMSY
= 1.06. Using the revised indices, the fishing mortality rates on
fully recruited fish in 2001 remained 0.24 yr-1, and the estimated
spawning biomass increased slightly to 5,300 mt. The revised
biomass-based benchmarks were F2001/FMSY = 0.72 and SSB2001/SSBMSY
= 0.99, supporting the same conclusion that the stock was neither
undergoing overfishing nor overfished.
The retrospective analyses using terminal years of 1998, 1999,
2000, and 2001 did not indicate that the models consistently
overestimated or underestimated either the fishing mortality rates
in the last year or the spawning biomass.
Landings of yellowtail snapper differ widely by subregion.
Yellowtail snapper were rarely landed north of Florida's Palm Beach
County on the Atlantic coast. From Palm Beach County south through
Miami-Dade County, yellowtail snapper were consistently landed;
however, in all three fishing sectors the majority of landings came
from the Florida Keys. The fishers from counties north of the Keys
on the gulf side also rarely landed yellowtail snapper. This
assessment focused on Southeast Florida (Palm Beach through
Miami-Dade counties) and the Florida Keys because of the
concentration of landings in those two subregions. The geographical
distribution of yellowtail snapper landings reflects the
distribution of coral reefs in Florida. Effort in terms of fishing
trips was proportionately higher in Southeast Florida than in the
Keys but there were still more trips in the Keys. Also, in all
three sectors, the catch rates were higher in the Florida Keys than
in Southeast Florida.
There was high compliance with the 12-inch minimum size (305
mm); only 3% of the commercial landings, 5% of the recreational
landings, and 2% of the headboat landings in the Atlantic region
were under the limit. In the Keys, the compliance was also high;
only 2% of the commercial landings, 4% of the recreational
landings, and 3% of the headboat landings were under the limit.
While we evaluated the 10-fish aggregate limit by assuming that all
of the snappers were yellowtail snapper, most of the recreational
anglers caught less than two fish per trip. Only 0.2% of the
anglers in the Atlantic region and 1.3% of the anglers in the Keys
exceeded 10 fish per trip.
For other information:
Stock assessments for finfish and invertebrate
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