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The growth rate of a fish is a fundamental function used in stock assessments to estimate the population size and the fishery pressure affecting the species. There has been recent debate within the stock assessment community regarding which type of growth model best represents the true growth rate of yellowfin tuna (Thunnus albacares), in the Atlantic Ocean; specifically, should assessments use a traditional von Bertalanffy growth curve or a so-called “two-stanza” growth curve, which combines one growth rate for smaller individual tuna and another for larger sizes. Using a simulated population created with the software R, both growth models were compared under different scenarios. The first part of this thesis examines the effect of different fisheries and their associated gear selectivity. Purse seine, baitboat and pelagic longline fisheries, which target yellowfin tuna in the Atlantic Ocean, were incorporated into the analysis. The second part looks at different sources of variability that occur either in nature (observation error) or in the process of analysis (process error). These include different growth variation, looking only at the fast growing young fish and using a set birth date versus a spawning period. These scenarios were used to determine if the sample, derived from a fishery-dependent sample, reflects the true population. Three populations of yellowfin tuna were created: an un-fished population, a fished population from stock assessment data, and a fished population from simulation software called ‘Population Simulator’. These populations were all analyzed for the different scenarios as well as the different fisheries. The final part of this thesis looks at three similar tuna species; skipjack (Katsuwonus pelamis), bigeye (Thunnus obesus ) and albacore (Thunnus alalunga). The same scenarios and gear selectivities were applied to these tuna species. The results of this study showed that the two-stanza growth curve was not a better fit for yellowfin tuna population in the Atlantic Ocean than the traditional von Bertalanffy growth curve. There were several scenarios that favored the two-stanza growth curve, but either it was a sample population not representative of the Atlantic Ocean population, or the two-stanza had no initial growth rate, making it the same as a traditional growth curve. Based on these results, it was evident that the traditional von Bertalanffy growth curve was the more accurate growth curve to use for yellowfin tuna in the Atlantic Ocean and it is recommended that this growth curve be used in the stock assessments going forward.

CoordenaÃÃo de AperfeÃoamento de Pessoal de NÃvel Superior / A sardinha-bandeira, Opisthonema oglinum, pertence à famÃlia Clupeidae da ordem Clupeiformes sendo uma das cinco espÃcies do gÃnero Opisthonema. No Estado do Cearà sua captura se estende ao longo de toda a costa, durante todo o ano, com as melhores pescarias ocorrendo entre maio e dezembro. O presente trabalho teve como objetivo determinar a estrutura da populaÃÃo, a relaÃÃo peso/comprimento da sardinha-bandeira e a seletividade das redes-de-espera utilizadas na sua captura na Praia da Caponga, municÃpio de Cascavel, CearÃ, Brasil. Os dados de comprimento total (cm) e peso individual (g) foram obtidos de dois grupos, cada um com 780 indivÃduos, capturados por redes-de-espera com malhas de 5 cm e 6 cm entre nÃs opostos, no perÃodo de agosto a outubro de 2014. Esses dados foram descritos em histogramas de frequÃncia de comprimento e peso, sendo mais representativas as classes de 21,6 cm (24,23%) e de 87,0 g (32,31%) para a malha de 5 cm e de 25,7 cm (23,08%) e de 117,9 g (26,03%), para a de 6 cm. Quando todos os indivÃduos foram reunidos em um Ãnico grupo (1.560), sem distinÃÃo do tamanho da malha, as classes mais representativas foram a de 23,0 cm de comprimento (18,59%) e a de 105,5 g de peso (20,38%). As equaÃÃes que representaram as relaÃÃes peso/comprimento foram as seguintes: y=0,016 x^2,7636 (780 indivÃduos capturados com a malha de 5 cm); y=0,031 x^2,5635 (780 indivÃduos capturados com a malha de 6 cm); e y=0,021 x^2,6828 (1.560 indivÃduos). Para o estudo da seletividade foram analisados todos os exemplares de sardinha-bandeira (1.560) capturados com as redes-de-espera de malha de 5 cm e 6 cm. O comprimento mÃdio de seleÃÃo foi de 23,7 cm para a malha de 5 cm e de 28,5 cm para a de 6 cm. A amplitude de seleÃÃo foi de 23,7  7,9 cm e de 28,5  7,9 cm para as malhas de 5 cm e 6 cm, respectivamente. As equaÃÃes das curvas de seleÃÃo das redes-de-espera para os dois tamanhos de malha foram obtidas, e a probabilidade de capturar indivÃduos com comprimento inferior ao mÃnimo, estabelecido em 15 cm, foi muito baixa e correspondeu a 1,32% e 0,03% para as malhas de 5 cm e de 6 cm, respectivamente. Esta informaÃÃo à de grande valia para a manutenÃÃo do estoque da sardinha-bandeira capturada na Praia da Caponga, CearÃ. / Atlantic thread-herring, Opisthonema oglinum, belongs to the family Clupeidae and to the order Clupeiformes. It is one of the five species of the genus Opisthonema. In the state of CearÃ, it is caught along the entire coast, throughout the year, with the best fisheries occurring between May and December. This work aimed to study the population structure, the length/weight relationships of the Atlantic thread-herring and to determine gear selectivity of drift nets used for catching them in Caponga Beach, Cascavel, CearÃ, Brazil. Data of total length (cm) and individual weight (g) were obtained from two groups, each with 780 individuals captured by drift nets with meshes of 5 cm and 6 cm, from August to October 2014. These data are showed as histograms. The most representative classes of length and weight were 21.6 cm (24.23%) and 87.0 g (32.31%) for 5 cm mesh, while for the 6 cm mesh, the main classes were 25.7 cm (23.08%) and 117.9 g (26.03%). When all data was combined into a single group (1,560), without the mesh size distinction, the most representative classes were 23.0 cm long (18.59%) and 105.5 g in weight (20.38%). The equations representing the weight/length ratios were as follows: y=0.016 x^2.7636 (780 individuals captured with a mesh of 5 cm); y=0.031 x^2.5635 (780 individuals caught with a mesh of 6 cm); and y=0.021 x^2.6828 (1,560 individuals). To study the selectivity, 1,560 Atlantic thread-herring specimens captured with drift nets of 5 cm and 6 cm were analyzed. The selection mean length was 23.7 cm for 5 cm mesh and 28.5 cm for 6 cm mesh. The range of selection was 23.7  7.9 cm and 28.5  7.9 cm for 5 cm mesh and 6 cm mesh, respectively. The equations of the selection curves for the two mesh sizes were obtained, and the probability of capturing individuals with length less than the established minimum of 15 cm was very low, amounting to 1.32% and 0.03% for the meshes of 5 cm and 6 cm, respectively. This information is of great value to maintain the stock of Atlantic thread-herring in Caponga Beach, CearÃ.