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Author Deleveaux, Vallierre K. W
Title On the growth and exploitation pattern effects on the recuperation from overfishing of the red snapper, Lutjanus campechanus, in the Gulf of Mexico
book jacket
Descript 131 p
Note Source: Dissertation Abstracts International, Volume: 73-10, Section: B, page:
Adviser: Nelson M. Ehrhardt
Thesis (Ph.D.)--University of Miami, 2012
Due to the very long history of high exploitation levels in the red snapper fishery (>130 yrs) there was a need to assess whether the minimum size regulations were serving their intended purpose of protecting the spawning stock, reducing the probability of recruitment failure and increasing the yield from the stock. To meet this objective a good descriptor of red snapper growth was required in order to simulate the existent exploitation pattern of the gears involved in the fisheries. An extensive analysis of red snapper growth was conducted using data supplied by the National Marine Fisheries Service (NMFS) Panama City, Florida Laboratory. The data set (database 1 (DB1)) contained information on otolith ageing and the annuli measurements made along the dorsal side of the sulcus acousticus as well as total length measurements collected from 370 fish between age 2 and 17. Data for the previous growth studies on red snapper, which were collected from the directed fisheries, indicated an underrepresentation of several age categories of fish. Due to the selectivity of the gears a major portion of the age two fish and even possibly some segment of the age three fish (smaller, slow-growing individuals) were not included in the data collection, therefore it is necessary to ensure that proportionality exists between the otolith and somatic growth. The results of the analysis here indicated that the back-calculated otolith radius-total length measurements were uncoupled for several age classes, meaning that the growth in total length was not proportional to the otolith growth in all age classes. This, therefore, indicated that the data was not suitable for providing a good descriptor of the growth of the red snapper when used in back-calculations of size-at-age
Several back-calculation models were selected to generate growth function parameters for comparison purposes. These models included several multiple regression models, the age-effects model (Morita & Matsuishi, 2001), a log-power model (Ehrhardt, 1992) and the Modified Fry model (Vigliola & Meekan, 2009). The result of each model was tested by comparing the mean of the observed lengths-at-age to the mean of the back-calculated lengths-at-age. It was found that all of the models used approximated the observed data as there was no significant difference between the observed mean lengths-at-age and the back-calculated mean-lengths-at-age at the 0.05% level
Next, how well the age structure of the samples represented the stock was investigated. It was found that the age two fish were underrepresented due to the selectivity of the fishing gears. Also, the larger and older fish in the size-at-age distributions were mostly absent, perhaps also due to fishing gear selectivity and the resulting exploitation pattern (Walter & Ingram, 2009). This problem was approached by treating the age two and the older age classes as containing missing data, which would suggest a slower growth rate for smaller fish and perhaps a larger maximum size for older fish. A Monte Carlo approach was used to generate experimental length-at-age data from the Modified-Fry model which would then be used to simulate an unbiased virgin red snapper length-at-age population structure based on the results of the Modified Fry model
Finally, to evaluate the appropriateness of the use of minimum size in the management framework to recuperate the red snapper stock it was necessary to consider the life history, the fishing mortalities due to the shrimp fishery, to the discards below the minimum size and to the closed season discards, and the temporal distribution of the fish stock. Several management scenarios, different minimum sizes under different levels of shrimp bycatch, were investigated that estimated the likelihood of red snapper recuperation to the management goal of 20% SPR by 2032. The results indicated that only in the complete absence of any shrimp fishery bycatch would the SPR approach 18.5%, still short of the goal, regardless of any minimum size selected. At the target 50% shrimp bycatch reduction level the SPR is only projected to increase to about 7.2%, regardless of the minimum size implemented. These results show the minimum size regulations to be ineffective in meeting their intended goal of protecting the spawning stock for red snapper in the Gulf of Mexico. (Abstract shortened by UMI.)
School code: 0125
Host Item Dissertation Abstracts International 73-10B
Subject Agriculture, Fisheries and Aquaculture
Alt Author University of Miami. Marine Biology and Fisheries (Marine)
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