SIZE COMPOSITION OF BLUE AND WHITE MARLINS TAKEN IN SELECTED FISHERIES IN THE WESTERN NORTH ATLANTIC

Transcription

1 SCRS/2000/060 Col. Vol. Sci. Pap. ICCAT, 53: (2001) SIZE COMPOSITION OF BLUE AND WHITE MARLINS TAKEN IN SELECTED FISHERIES IN THE WESTERN NORTH ATLANTIC C. Phillip Goodyear 1 and Freddy Arocha 2 SUMMARY Size frequencies of catches represent a useful adjunct to catch, effort and abundance information for stock assessment. Size frequencies of blue and white marlin (Makaira nigricans, and Tetrapturus albidus, respectively) have been collected at U.S. recreational tournaments since The U.S. National Marine Fisheries Service (NMFS) Marine Recreational Fishery Statistics Survey (MRFSS), and the NMFS Large Pelagic Survey have made limited additional observations of the U.S. recreational marlin catch during dockside interviews of fishermen since 1982 and 1984, respectively. Other size data for marlin are available for U.S. and Venezuelan longline fisheries. These include measurements taken by observers on Venezuelan longline vessels since 1987, and on U.S. vessels since These data are supplemented with dockside samples of Venezuelan longline and artisanal gillnet billfish landings beginning in Length frequencies constructed from these data showed increasing mean sizes in the U.S. recreational fisheries in recent years. This trend is the result of the implementation of minimum size regulations that truncated the size distribution of fish landed by U.S. recreational fishermen. This trend is not reflected in the samples from the other fisheries. Sex ratios for both species change from predominately male, or unknown sex at smaller sizes to predominantly female at larger sizes. The examination of the size frequency data from these surveys emphasizes the need too address the sexual dimorphism issue in any analyses that attempt to interpret changes in observed size frequencies in the context of stock assessment RESUMEN Las frecuencias de tallas de las capturas representan una información añadida a la captura, esfuerzo y abundancia que resulta útil para evaluar el stock. A partir de 1972, en los campeonatos de la pesquería de recreo de Estados Unidos se han obtenido frecuencias de tallas de la aguja azul y aguja blanca (Makaira nigricans y Tetrapturus albidus, respectivamente). El U.S. National Marine Fisheries Service (NMFS), Marine Recreational Fishery Statistics Survey (MRFS) y el NMFS Large Pelagic Survey, han realizado un número limitado de observaciones complementarias de la captura de marlines de la pesquería de recreo de Estados Unidos en el curso de entrevistas con los pescadores en los muelles, desde 1982 y 1984, respectivamente. Se dispone de otros datos de tallas para marlines de las pesquerías palangreras venezolanas y estadounidenses. Incluyen mediciones efectuadas por observadores a bordo de palangreros venezolanos a partir de 1987 y a bordo de barcos estadounidenses a partir de Estos datos se complementan con muestras tomadas en los muelles en los desembarques de marlines del palangre y de la pesquería artesanal de redes de enmalle de Venezuela, a partir del año Las frecuencias de talla obtenidas a partir de estos datos mostraban un aumento en las tallas medias en las pesquerías de recreo estadounidenses en los últimos años. Esta tendencia es el resultado de la implementación de regulaciones sobre talla mínima, lo cual ha truncado la distribución de tallas de los peces desembarcados por dichas pesquerías de recreo. Esta tendencia no se observa en las muestras procedentes de otras pesquerías. Las proporciones de sexos para ambas especies cambian, desde una mayoría de peces machos, o de sexo desconocido en las tallas menores, a una mayoría de peces hembra en las tallas superiores. El examen de los datos de frecuencias de tallas obtenidos en estas encuestas, ponen de relieve la necesidad de estudiar el dimorfismo sexual en cualquier análisis que intente interpretar los cambios en las frecuencias de tallas observadas en el contexto de la evaluación de stock Ridgewood Road, Key Biscayne, FL USA, phil_goodyear@ .msn.com. 2 Instituto Oceanografico de Venezuela, Universidad de Oriente, Cumaná, Venezuela. 249

2 RÉSUMÉ La fréquence des tailles dans les prises est un complément utile aux informations sur la capture, l effort et l abondance aux fins des évaluations de stock. Les fréquences de taille du makaire bleu (Makaira nigricans) et du makaire blanc (Tetrapturus albidus) sont collectées depuis 1972 à l occasion des championnats de pêche sportive. Le U.S. National Marine Fisheries Service (NMFS), la Marine Recreational Fishery Statistics Survey (MRFSS) et la Large Pelagic Survey du NMFS ont receuilli à quai auprès des pêcheurs un nombre limité d observations sur la capture de makaires par la pêche sportive américaine, respectivement depuis 1982 et D autres données sur la taille des makaires sont disponibles pour les pêcheries palangrières des Etats-Unis et du Venezuela. Elles comprennent des mensurations relevées par des observateurs à bord de palangriers vénézuéliens depuis 1987, et de bateaux américains depuis Ces données sont complémentées par des échantillons, prélevés à quai depuis 1987, des débarquements vénézuéliens, palangriers et artisanaux d istiophoriés pris au filet maillant. Les fréquences de taille élaborées à partir de ces données révèlent des tailles moyennes croissantes ces dernières années chez les poissons débarqués par la pêche sportive américaine. Cette tendance n est pas reflétée dans les échantillons des autres pêcheries. Le sex ratio des deux espèces évolue, d une majorité de mâles, ou de sexe inconnu, aux petites tailles à une majorité de femelles aux plus grandes tailles. L examen des données de fréquence de tailles dans ces enquêtes met l accent sur la nécessité d aborder la question du dimorphisme sexuel dans toute analyse tentant d interpréter les modifications des fréquences de taille observées dans le contexte de l évaluation des stocks. KEYWORDS Sport fishing, Longlining, Pelagic fisheries, Fish catch statistics, Size composition, Fishery surveys, Fishery regulations INTRODUCTION Changes in the size frequencies of catches can be useful indicators of changes in the age structure of stocks, particularly if the selectivities of the fisheries are constant. Because changes in the age structure of a stock reflect the dynamics of recruitment and mortality, size frequency data represent a useful adjunct to catch, effort and abundance information for use in stock assessments. In the western north Atlantic, size frequencies of blue and white marlin (Makaira nigricans, and Tetrapturus albidus, respectively) have been routinely collected at U.S. billfish tournaments monitored by U.S. National Marine Fisheries Service (NMFS) since Similar but less extensive data have been taken as a part of routine monitoring of U.S. recreational and commercial fisheries. These include the NMFS Marine Recreational Fishery Statistics Survey (MRFSS) and the Large Pelagic Survey (LPS) that sample the U.S. recreational catch, and also data collected by observers aboard U.S. longline fishing vessels. In addition to these data, the ICCAT enhanced research program for billfish sponsored observer and dockside sampling of Venezuelan longline and artisanal gillnet and longline fisheries since This paper presents the annual size frequency data collected by these various programs, and where possible apportions the catch at size by sex. METHODS Data from each the various research programs were available as one or more computer files. Observations of fish length were extracted and sorted by year, sex and length. The sex categories included males (M), females (F), and unknown (U). For blue marlin, the lengths were sorted into 10-cm bins. For white marlin, the lengths were sorted into 5-cm bins. Where possible, all length measurements were converted from measured units to lower jaw fork length (LJFL). Conversions were made from weight, eye orbit fork length (EOFL), pectoral anal length (PAL), pectoral fork length (PFL), and total length (TL) using the equations developed by Prager et al (1995). MRFSS and LPS measured and recorded length only as fork length (FL), and these units could not be converted to LJFL with the existing relation- 250

3 ships. Consequently, lengths from these two sources are reported in units of FL rather than LJFL. For these two sources only, missing measures of FL were estimated from fish weights using a weight-length equation derived from data collected by these two programs. RESULTS Blue Marlin The number of observations of blue marlin sizes and by source and sex are given in Table 1. The most extensive sampling is associated with the U.S. tournament survey conducted by NMFS that began in 1972 and continued to the present. Length frequencies by year and sex for this survey are presented in Figure 1. The majority of sampled blue marlin were identified by sex. Inspection of the data in Figure 1 indicates that the smaller fish tended to be males while the larger fish were predominantly female. Also, the number of fish below about 220 cm LJFL that were brought to port and available for inspection declined after 1988 which coincided with the implementation of the minimum size provisions of the U.S. Billfish Management Plan. In addition to the tournament survey, the sizes of blue marlin harvested by recreational fishermen have been measured during the intercept portions of the NMFS LPS and MRFSS surveys. These data are presented in Figures 2 and 3. In contrast to the other data collection programs LPS and the MRFSS only measure length as fork length; consequently, data in these two plots are not strictly comparable to those collected in the other surveys. Also, information on the sex of the measured fish was not recorded. The main feature of the information in Figures 2 and 3 is the sparseness of the data, a consequence of the fact that blue marlin are only sporadically encountered in fishermen interviews. However, it is also apparent that the number of blue marlin measured by these two surveys has declined in recent years. Each survey encountered only eight specimens returned to port by fishermen during the six-year period 1994 through At least a part of this decline compared to earlier years is likely the result of an increased conservation ethic among anglers, and conservation measures associated with the U.S. Billfish Management Plan. Observers have accompanied some U.S. commercial longline vessel trips since 1985, but the program began in earnest in Lengths of blue marlin were either estimated or actually measured depending on the condition of the fish. Length frequencies of all blue marlin for which length information was recorded are presented in Figure 4; and the length frequencies for only fish that were actually measured are given in Figure 5. It is obvious from the comparison of these two figures that measurements were not taken for most blue marlin observed during these longline trips. Size frequencies of blue marlin observed during port sampling of the Venezuelan longline and artisanal (gillnet and longline) fisheries are presented in Figure 6. These data extend from 1987 through Almost no sex information was collected for most fish during the early years of the shore-based sampling, mostly due to the fact that these specimens came from the gillnet artisanal fishery, where fish were landed gutted most of the time. In contrast, most of the fish in the observer data were examined for size and sex. It appears that most of the fish taken in the artisanal fisheries are males. This trend also appears to hold true for the data collected by the observers on the Venezuelan longline vessels Figure 7. White Marlin The number of observations of white marlin sizes and by source and sex are given in Table 2. As with blue marlin, the most extensive sampling is associated with the U.S. tournament survey conducted by NMFS which began in Length frequencies by year and sex for this survey are presented in Figure 8. In most years the majority of observed white marlin were identified by sex. Inspection of the data in Figure 8 indicates that, as was true with blue marlin, the smaller fish tended to be males while the larger fish were predominantly females. Also, the number of fish below about 160 cm LJFL that were brought 251

4 to port and available for inspection declined in the late 1980 s which coincided with the implementation of the minimum size provisions of the U.S. Billfish Management Plan. In addition to the tournament survey, the sizes of white marlin harvested by recreational fishermen have been measured during the intercept portions of the NMFS LPS and MRFSS surveys. These data are presented in Figures 9 and 10. As noted in the discussion of blue marlin, LPS and the MRFSS only measure length as fork length; consequently, data in these two plots are not strictly comparable to those collected in the other surveys. Also, information on the sex of the measured fish was not recorded. The main feature of the information in Figures 9 and 10 is the sparseness of the data, particularly in recent years. This is partly a consequence of the fact that white marlin have always been a rare event species in the MRFSS, and have become increasingly rare in the LPS, with only 12 specimens observed from 1994 through However, it is also apparent that the number of white marlin measured by MRFSS has also declined in recent years, with only 7 specimens measured from 1990 through As with blue marlin, at least a part of this decline compared to the earlier years of the surveys is likely the result of both an increased conservation ethic among anglers, and conservation measures associated with the U.S. Billfish Management Plan. U.S. observers have covered a portion of U.S. commercial longline trips since As with blue marlin, lengths of white marlin were either estimated or actually measured depending on the condition of the fish. Length frequencies of all white marlin for which length information was recorded are presented in Figure 11, and the length frequencies for only fish that were actually measured are given in Figure 12. It is obvious from the comparison of these two figures that measurements were not taken for most white marlin observed during these longline trips. Size frequencies of white marlin observed during port sampling of the Venezuelan longline and artisanal (gillnet and longline) fisheries are presented in Figure 13. These data extend from 1987 through Although sex information was not collected for most fish until 1997 (primarily because most of the white marlin are landed gutted by the artisanal longline fishery), it appears that most of the fish taken in these fisheries are males. This trend also appears to hold true for the data collected by the observers on the Venezuelan longline vessels in Figure 14. However, it is interesting to note that white marlin above about 175 cm were rarely encountered in the Venezuelan observer data, which corresponds to the size that females tend to dominate the sex ratio in the U.S. tournament data. DISCUSSION Since fish size increases with age, size frequency data are often useful for examining age structure in fish stocks, and particularly changes in age structure with time that may reflect changes in mortality. Generally, reductions in mean size or the abundance of large fish tend to reflect lower survival to older age, which is an expected outcome of increased fishing mortality (or vice versa). The size frequency data examined in this paper indicates that shifts in the size composition of both blue and white marlin are associated with changes in the sex composition of the samples. This is an expected outcome of the strong sexual dimorphism in growth that exists in these species (Wilson 1984; Wilson, et al.1991). As a consequence, it is evident for these species, that changes in observed length frequencies through time and/or space may reflect changes in sex composition as well as possible changes in survivorship. Consequently, analyses that attempt to quantify changes in mortality from observed changes in size compositions with time must first nullify any effects of changes in the sex composition of the observations. Likely, such a task will require partitioning size data by sex for any meaningful analysis. 252

5 ACKNOWLEDGEMENTS Thanks are extended to the many NMFS and Venezuelan scientists, who collected, entered, maintained and provided the data bases from which these data were extracted. The Billfish Foundation supported the senior author s contribution to this work. REFERENCES PRAGER, M. H., E.D. Prince, and D.W. Lee Empirical length and weight conversion equations for blue marlin, white marlin, and sailfish from the North Atlantic Ocean. Bull. Mar. Sci. 56(1): WILSON, C. A Age and growth aspects of the life history of billfishes. Ph.D. Dissertation. University of South Carolina, USA..180 pages. WILSON, C. A., J. M. Dean, E. D. Prince, and D. W. Lee An examination: of sexual dimorphism in Atlantic and Pacific blue marlin using body weight, sagittae weight, and age estimates. J. Exper. Mar. Biol. Ecol. 151: Table 1. Numbers of observations of blue marlin sizes by source and sex. Observations Observations Source Year Total Male Female Source Year Total Male Female USA TOURN USA LPS USA TOURN USA LPS USA TOURN USA LPS USA TOURN USA LPS USA TOURN USA LPS USA TOURN USA LPS USA TOURN USA LPS USA TOURN USA LPS USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN VEN OBSER USA TOURN VEN OBSER USA TOURN VEN OBSER USA TOURN VEN OBSER USA TOURN VEN OBSER USA TOURN VEN OBSER USA MRFSS VEN OBSER USA MRFSS VEN OBSER USA MRFSS VEN OBSER USA MRFSS VEN OBSER USA MRFSS VEN OBSER USA MRFSS VEN OBSER USA MRFSS VEN OBSER USA MRFSS VEN SHORE USA MRFSS VEN SHORE USA MRFSS VEN SHORE USA MRFSS VEN SHORE USA MRFSS VEN SHORE USA MRFSS VEN SHORE USA MRFSS VEN SHORE USA MRFSS VEN SHORE USA MRFSS VEN SHORE USA LPS VEN SHORE USA LPS VEN SHORE USA LPS VEN SHORE USA LPS VEN SHORE USA LPS

6 Table 2. Numbers of observations of white marlin sizes by source and sex. Observations Observations Source Year Total Male Female Source Year Total Male Female USA TOURN USA LPS USA TOURN USA LPS USA TOURN USA LPS USA TOURN USA LPS USA TOURN USA LPS USA TOURN USA LPS USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN USA OBSER USA TOURN VEN OBSER USA TOURN VEN OBSER USA TOURN VEN OBSER USA TOURN VEN OBSER USA TOURN VEN OBSER USA TOURN VEN OBSER USA TOURN VEN OBSER USA TOURN VEN OBSER USA MRFSS VEN OBSER USA MRFSS VEN OBSER USA MRFSS VEN OBSER USA MRFSS VEN OBSER USA MRFSS VEN OBSER USA MRFSS VEN SHORE USA MRFSS VEN SHORE USA MRFSS VEN SHORE USA MRFSS VEN SHORE USA MRFSS VEN SHORE USA MRFSS VEN SHORE USA MRFSS VEN SHORE USA LPS VEN SHORE USA LPS VEN SHORE USA LPS VEN SHORE USA LPS VEN SHORE USA LPS VEN SHORE USA LPS VEN SHORE USA LPS Figure 1. Size frequencies of blue marlin by sex sampled from recreational fishing tournaments frequented by U.S. recreational fishermen ( ). 254

7 Figure 2. Size frequencies of blue marlin sampled as part of the U.S. National Marine Fisheries Service Large Pelagic Survey (LPS) of recreational fishermen. Figure 3. Size frequencies of blue marlin sampled as a part of the U.S. National Marine Fisheries Service Marine Recreational Fisheries Statistics Survey (MRFSS). Figure 4. Size frequencies of blue marlin by year and sex based on lengths measured and estimated by U.S. observers aboard U.S. commercial longline vessels ( ). Figure 5. Size frequencies of blue marlin by year and sex from fish measured by U.S. observers aboard U.S. commercial longline vessels ( ). Figure 6. Size frequencies of blue marlin by sex from port samples of Venezuelan commercial longline and artisanal gillnet landings ( ). Figure 7. Size frequencies of blue marlin by sex sampled by Venezuelan observers aboard commercial longline vessels ( ). 255

8 Figure 8. Size frequencies of white marlin by sex sampled from recreational fishing tournaments frequented by U.S. recreational fishermen ( ). Figure 9. Size frequencies of white marlin sampled as part of the U.S. National Marine Fisheries Service Large Pelagic Survey (LPS) of recreational fishermen. Figure 10. Size frequencies of white marlin sampled as a part of the U.S. National Marine Fisheries Service Marine Recreational Fisheries Statistics Survey (MRFSS). Figure 11. Size frequencies of white marlin by year and sex based on lengths measured and estimated by U.S. observers aboard U.S. commercial longline vessels ( ). Figure 12. Size frequencies of white marlin by year and sex from fish measured by U.S. observers aboard U.S. commercial longline vessels ( ). 256

9 Figure 13. Size frequencies of white marlin by sex from port samples of Venezuelan commercial longline and artisanal gillnet landings ( ). Figure 14. Size frequencies of white marlin by sex sampled by Venezuelan observers aboard commercial longline vessels ( ). 257