Improving the Collection, Management, and Use of Marine Fisheries Data (2000) / Chapter Skim
Currently Skimming:
Summer Flounder: Review and Insights
Summer Flounder: Review and Insights
Pages 20-58
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From page 20... ...
Commercial fishermen expressed a variety of views that indicated a general lack of trust in the management of summer flounder and other marine fisheries. Many recreational fishermen and their representatives were invited to the committee's meetings.
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From page 21... ...
· Many fishermen question a number of assumptions about the biology and population dynamics used in specific stock assessments (e.g., in the case of summer flounder, this includes the assumed level of natural mortality and the assumption of constant selectivity for the older age groups)
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From page 22... ...
Considered lie-in-wait predators, they eat a large variety of prey, usually smaller fish, crabs, and shrimp. Commercial catches of summer flounder along the Atlantic Coast have fluctuated between 2 and 18 million kg over the past 50 years.
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From page 23... ...
The most important problem is that a combined stock may appear to be in reasonable shape even though some component is being fished to extinction. Combining sub-stocks may also hinder stock assessments if growth, recruitment, natural mortality, or fishing mortality differ substantially among the sub-stocks of a combined stock.
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From page 24... ...
What are the implications for stock assessments and management if the unit stock assumption is wrong? If separate stocks are fished differently, the fishing mortality rate estimated for the combined stock will not be applicable to the separate stocks.
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From page 25... ...
What natural mortality rate is appropriate to use in summer flounder assessment models? 25 cases, M is assumed to be the difference between total mortality and fishing mortality.
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From page 26... ...
IMPROVING THE COLLECTION, MANAGEMENT, AND USE OF MARINE FISHERIES DATA .. o 0 0.5 1 1.5 2 Fishing Mortality M 0.1 ~ ~ ~ ~ 0.2 0.3 FIGURE 2-2 Summer flounder yield per recruit for three levels of natural mortality.
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From page 27... ...
Spawning stock biomass is similar in both runs in 1997 but is higher for M = 0.1 in 1998. This is due to the larger reduction in fishing mortality rate.
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From page 28... ...
See Box 1-2 for other definitions. The assumed value of M not only affects estimates of fishing mortality, survivorship, and biomass, it also affects biological reference point estimates.
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From page 29... ...
However, mortality at age should be greater for females than for males, until both are equally well retained by trawl nets in commercial fisheries and hooks in recreational fisheries. In turn, this greater mortality for females would lower the estimated spawning potential of the stock, decrease the effective spawning stock biomass, and increase the effective fishing mortality.
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From page 30... ...
What are the appropriate survey and commercial catchabilities of summer flounder? Commercial fishermen believe that NEFSC surveys fail to fish appropriately in the survey strata areas and depth where the industry believes large summer flounder are found.
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From page 31... ...
The exploitation pattern, however, may decline at larger sizes if larger fish are better able to escape the gear or the gear is used more intensely in areas where smaller fish live. Thus, if at least some of the older ages are caught and the exploitation pattern can be estimated, assessment models can correct for changes in the proportion of different ages caught.
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From page 32... ...
Such default assumptions also may lead to overestimation of the status quo fishing mortality; it is apparent from the VPA results that catchability is assumed constant for ages 3, 4, and the 5+ group. Because recreational catch accounts for about half of the total catch, the catchability of the recreational fishery and its effect on population estimates also is important.
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From page 33... ...
Thus, how critical a factor (such as the right exploitation curve) is to management might be measured by asking how it changes the ratio between the current and desired fishing mortality rates.
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From page 34... ...
It also illustrates that the fishing mortality rate value adopted for age 4 (the last true age in the catch-atage data) has to be applied to age 5 fish and older.
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From page 35... ...
NOTE: The fishing mortality rate (not selection) of 0.8 on age-4 and older fish stems from the fact that the age-4 mortality is estimated as the average of the mortality rate on ages 1-3 and that on older ages set equal to the age4 fish.
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From page 36... ...
Moreover, at age 6, the fish are a substantial proportion of their maximum size and weight. These fish may reasonably be expected to behave like even older fish and be equally catchable and may have the same fishing mortality rate.
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From page 37... ...
If older fish have a lower fishing mortality rate than is supposed in current assessments, they would have a higher average age than predicted. Thus, tagged fish would continue to be returned for a longer period than would have been predicted under the current assessment.
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From page 38... ...
Because the debate about the population structure centers in the offshore area, this area should be targeted for special tagging and egg survey studies, as well as other observations designed to determine if larger, older summer flounder are more abundant offshore than expected. The significant contribution of the recreational fishery to the fishing mortality rate makes the actual exploitation patterns of recreational anglers and model assumptions about these patterns more important in summer flounder stock assessments.
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From page 39... ...
· spawning stock biomass estimates are also decreased for the same reason, as is the 1998 FmaX TAC and the 1997 status quo TAC. · the estimate of fishing mortality rate is higher using the NEFSC ages only.
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From page 40... ...
Are effort data used appropriately and are the effects of effort changes incorporated properly? In many fishery assessments, commercial fishing effort data and the associated commercial catch data are the primary means of establishing the trends in fishing mortality rate and population size and hence in establishing the current state of the stock and the current level of TACs.
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From page 41... ...
each year. This is necessary because undisclosed discards or catch may cause an underestimation of fishing mortality rates, and an
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From page 42... ...
In practice, it is this latter percentage that is of far greater importance because it represents a fishing mortality rate of up to about 0.4 (Figure 2-5~; only a small number of age-0 fish are caught in the fishery, so the high discard rate is not as significant to the population. The discard mortality is assumed to be 80 percent for the commercial fishery (based on commercial advice to NMFS)
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From page 43... ...
_ o ~5 a, (a m LL 0.5 0.4 0.3 0.2 o _ _ — — — — — — — — — — — 1 . x9~ x9~ x9~ x9~9 x99\ x99~ x99 Year FIGURE 2-5 Discard fishing mortality for age-O and age-1 fish.
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From page 44... ...
Recreational fishing used to be a minimal source of fishing mortality, but current estimates show that anglers contribute substantially to fishing mortality in several fisheries, including summer flounder. Summer flounder is one of several major flatfish species found along the Atlantic coast of the United States (others include winter flounder, American plaice, yellowtail flounder, and witch flounder)
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From page 45... ...
The South Atlantic region includes landings in North Carolina, South Carolina, Georgia, and Florida. PSE = proportional standard error, which is the standard error of an estimate as a percentage of the estimate.
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From page 46... ...
The typical practice in recreational fisheries that have important commercial components is to obtain length data from recreational access-site surveys and to estimate catch at age from age-length keys derived from the commercial catch. Hence, if there are problems with ageing the commercial catch for a species, the same problems will exist for the recreational catch.
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From page 48... ...
Again, any lack of fit is ascribed to sampling error. The same approach also can be adopted for catch-at-age data using age and year-class factors to explain sources of systematic variation seen in the survey catch rate data and a year factor (to account for annual changes in fishing mortality rate as well as systematic changes through time)
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From page 49... ...
The weight-at-age data seem to have lower standard deviations of log-transformed data, which is adequate for most assessments. Note, in particular, on a survey-by-survey basis, that the NEFSC fall and winter survey estimates are less variable than those of the state surveys.
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From page 50... ...
n/a 0.30 0.21 0.25 0.34 0.27 NOTE: Cells with "n/a" are for those ages with no available data. CTDEP = Connecticut Department of Environmental Protection; MADMF = Massachusetts Department of Marine Fisheries; NEFSC = Northeast Fisheries Science Center; NJDF = New Jersey Department of Fisheries.
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From page 51... ...
The ADAPT model apparently does not use |0 Shrinkage is the term for a procedure that gives some weight to the assumption that fishing mortality rate (and in some models also population numbers) are unchanging through the recent past.
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From page 52... ...
Some investigation is possible using the wellknown properties of VPA estimates. Fishing mortality rates estimated by this approach for younger ages in earlier years can be regarded as being relatively unbiased by assumptions.
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From page 53... ...
end fate are the selectivity at age a and full recruitment fishing mortality at year t for the fleet and quad is the catchability at age of the survey. Consequently, an ANOVA applied to the log ratio should be able to distinguish a year effect Log(t)
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From page 54... ...
POSSIBLE IMPROVEMENTS TO THE SUMMER FLOUNDER DATA SETS Based on the answers to the questions in Box 2-1, a number of actions could be taken to improve data collection. Actions could include
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From page 55... ...
SUMMER FLOUNDER: REVIEW AND INSIGHTS 55 using tournament data, conducting social and economic studies of commercial and recreational fishermen, employing commercial fishermen as collectors, and improving observer program data. Any changes in the sampling intensity should be judged in the context of the contribution of specific data to the overall assessment and management of the species and the value of improving management of summer flounder in relation to the management of other species.
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From page 56... ...
56 lo lo s~equ~nN u°!
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From page 57... ...
A Role for Commercial and Recreational Fishermen in Data Collection The summer flounder fishery clearly demonstrates the need for better cooperation among recreational anglers, commercial fishermen, state agencies, and federal agencies in data collection so that data quality is improved and its credibility is enhanced. There is room for improvement from all sectors.
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From page 58... ...
Although commercial fishermen expressed concern that older fish were potentially missed by NEFSC trawl surveys, some of these fish are apparently distributed near shore, where they are captured in recreational fisheries, primarily in the summer. A comparison of such data with commercial catch data from the same area could be used to crosscheck data sources and assumptions.
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