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

SCRS/2000/060 Col. Vol. Sci. Pap. ICCAT, 53: 249-257 (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 1972. 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 1989. These data are supplemented with dockside samples of Venezuelan longline and artisanal gillnet billfish landings beginning in 1987. 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 1989. 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 1987. 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. 1 415 Ridgewood Road, Key Biscayne, FL 33149 USA, Email: phil_goodyear@email.msn.com. 2 Instituto Oceanografico de Venezuela, Universidad de Oriente, Cumaná, Venezuela. 249

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 1984. 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 1989. 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 1972. 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 1987. 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

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 1999. 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 1992. 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 1999. 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 1972. 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

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 1999. 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 1999. 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 1985. 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 1999. 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

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. 1995. Empirical length and weight conversion equations for blue marlin, white marlin, and sailfish from the North Atlantic Ocean. Bull. Mar. Sci. 56(1): 201-210. WILSON, C. A. 1984. 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. 1991. An examination: of sexual dimorphism in Atlantic and Pacific blue marlin using body weight, sagittae weight, and age estimates. J. Exper. Mar. Biol. Ecol. 151:209-225. 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 1972 106 61 38 USA LPS 1990 11 0 0 USA TOURN 1973 166 70 75 USA LPS 1991 7 0 0 USA TOURN 1974 221 110 89 USA LPS 1992 9 0 0 USA TOURN 1975 174 88 75 USA LPS 1993 4 0 0 USA TOURN 1976 144 56 83 USA LPS 1994 1 0 0 USA TOURN 1977 155 49 93 USA LPS 1995 1 0 0 USA TOURN 1978 136 58 54 USA LPS 1997 3 0 0 USA TOURN 1979 106 36 47 USA LPS 1998 3 0 0 USA TOURN 1980 353 79 183 USA OBSER 1985 14 0 0 USA TOURN 1981 569 131 267 USA OBSER 1986 22 0 0 USA TOURN 1982 384 112 203 USA OBSER 1987 7 4 0 USA TOURN 1983 807 287 285 USA OBSER 1989 7 0 0 USA TOURN 1984 623 201 320 USA OBSER 1990 27 0 0 USA TOURN 1985 568 171 325 USA OBSER 1991 7 0 1 USA TOURN 1986 389 144 199 USA OBSER 1992 59 0 0 USA TOURN 1987 277 60 177 USA OBSER 1993 172 7 14 USA TOURN 1988 400 38 181 USA OBSER 1994 124 2 3 USA TOURN 1989 118 20 72 USA OBSER 1995 181 7 5 USA TOURN 1990 140 13 93 USA OBSER 1996 138 10 2 USA TOURN 1991 121 8 106 USA OBSER 1997 174 12 6 USA TOURN 1992 128 9 82 USA OBSER 1998 65 3 0 USA TOURN 1993 299 83 194 USA OBSER 1999 136 4 2 USA TOURN 1994 132 16 97 VEN OBSER 1987 38 20 18 USA TOURN 1995 125 10 101 VEN OBSER 1988 13 9 3 USA TOURN 1996 105 4 89 VEN OBSER 1989 11 6 4 USA TOURN 1997 143 3 126 VEN OBSER 1990 33 21 6 USA TOURN 1998 94 12 71 VEN OBSER 1991 21 9 10 USA TOURN 1999 124 2 62 VEN OBSER 1992 76 35 34 USA MRFSS 1982 3 0 0 VEN OBSER 1993 82 36 42 USA MRFSS 1983 2 0 0 VEN OBSER 1994 201 109 85 USA MRFSS 1984 4 0 0 VEN OBSER 1995 316 217 93 USA MRFSS 1985 4 0 0 VEN OBSER 1996 157 101 49 USA MRFSS 1986 11 0 0 VEN OBSER 1997 132 102 19 USA MRFSS 1987 10 0 0 VEN OBSER 1998 187 152 35 USA MRFSS 1988 6 0 0 VEN OBSER 1999 65 50 15 USA MRFSS 1989 4 0 0 VEN SHORE 1987 6 0 1 USA MRFSS 1990 1 0 0 VEN SHORE 1988 363 64 2 USA MRFSS 1991 1 0 0 VEN SHORE 1989 266 154 6 USA MRFSS 1992 2 0 0 VEN SHORE 1990 355 43 46 USA MRFSS 1993 3 0 0 VEN SHORE 1991 285 36 45 USA MRFSS 1994 2 0 0 VEN SHORE 1992 456 15 14 USA MRFSS 1995 1 0 0 VEN SHORE 1993 495 27 51 USA MRFSS 1997 2 0 0 VEN SHORE 1994 549 40 55 USA MRFSS 1998 1 0 0 VEN SHORE 1995 54 22 18 USA LPS 1985 25 0 0 VEN SHORE 1996 9 4 0 USA LPS 1986 24 0 0 VEN SHORE 1997 165 64 44 USA LPS 1987 24 0 0 VEN SHORE 1998 651 169 161 USA LPS 1988 9 0 0 VEN SHORE 1999 2405 1118 749 USA LPS 1989 27 0 0 ---------------------------------------------------------------------------------------------- 253

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 1972 67 24 38 USA LPS 1992 5 0 0 USA TOURN 1973 61 16 31 USA LPS 1993 6 0 0 USA TOURN 1974 196 76 74 USA LPS 1994 2 0 0 USA TOURN 1975 154 69 65 USA LPS 1995 2 0 0 USA TOURN 1976 103 60 41 USA LPS 1997 5 0 0 USA TOURN 1977 59 16 35 USA LPS 1998 3 0 0 USA TOURN 1978 171 103 55 USA OBSER 1985 21 3 2 USA TOURN 1979 71 15 28 USA OBSER 1986 3 1 0 USA TOURN 1980 1345 173 293 USA OBSER 1987 37 2 1 USA TOURN 1981 837 109 261 USA OBSER 1989 5 0 0 USA TOURN 1982 473 80 220 USA OBSER 1990 87 0 0 USA TOURN 1983 880 155 291 USA OBSER 1991 26 1 0 USA TOURN 1984 642 134 262 USA OBSER 1992 128 3 5 USA TOURN 1985 327 73 190 USA OBSER 1993 371 41 47 USA TOURN 1986 210 35 124 USA OBSER 1994 172 9 9 USA TOURN 1987 201 37 76 USA OBSER 1995 359 37 41 USA TOURN 1988 140 23 78 USA OBSER 1996 124 25 12 USA TOURN 1989 88 44 37 USA OBSER 1997 268 45 25 USA TOURN 1990 52 8 31 USA OBSER 1998 108 17 21 USA TOURN 1991 99 41 43 USA OBSER 1999 255 21 22 USA TOURN 1992 61 13 40 VEN OBSER 1987 144 76 68 USA TOURN 1993 50 5 35 VEN OBSER 1988 60 39 21 USA TOURN 1994 47 7 31 VEN OBSER 1989 42 14 13 USA TOURN 1995 37 9 25 VEN OBSER 1990 67 28 19 USA TOURN 1996 41 6 29 VEN OBSER 1991 37 9 26 USA TOURN 1997 31 6 20 VEN OBSER 1992 88 36 48 USA TOURN 1998 17 1 2 VEN OBSER 1993 228 119 96 USA TOURN 1999 22 1 4 VEN OBSER 1994 337 130 143 USA MRFSS 1981 25 0 0 VEN OBSER 1995 722 325 376 USA MRFSS 1982 2 0 0 VEN OBSER 1996 188 106 67 USA MRFSS 1983 4 0 0 VEN OBSER 1997 369 258 93 USA MRFSS 1984 2 0 0 VEN OBSER 1998 282 177 101 USA MRFSS 1985 2 0 0 VEN OBSER 1999 115 68 47 USA MRFSS 1986 3 0 0 VEN SHORE 1987 3 0 0 USA MRFSS 1987 22 0 0 VEN SHORE 1988 526 0 4 USA MRFSS 1988 8 0 0 VEN SHORE 1989 320 0 0 USA MRFSS 1989 13 0 0 VEN SHORE 1990 546 56 38 USA MRFSS 1990 2 0 0 VEN SHORE 1991 2216 66 40 USA MRFSS 1991 3 0 0 VEN SHORE 1992 2958 29 32 USA MRFSS 1992 2 0 0 VEN SHORE 1993 2315 15 8 USA LPS 1985 77 0 0 VEN SHORE 1994 262 12 17 USA LPS 1986 61 0 0 VEN SHORE 1995 496 10 20 USA LPS 1987 119 0 0 VEN SHORE 1996 33 0 0 USA LPS 1988 44 0 0 VEN SHORE 1997 19 3 7 USA LPS 1989 107 0 0 VEN SHORE 1998 485 230 41 USA LPS 1990 79 0 0 VEN SHORE 1999 933 505 194 USA LPS 1991 33 0 0 ---------------------------------------------------------------------------------------------- Figure 1. Size frequencies of blue marlin by sex sampled from recreational fishing tournaments frequented by U.S. recreational fishermen (1972-1999). 254

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 (1989-1999). Figure 5. Size frequencies of blue marlin by year and sex from fish measured by U.S. observers aboard U.S. commercial longline vessels (1989-1999). Figure 6. Size frequencies of blue marlin by sex from port samples of Venezuelan commercial longline and artisanal gillnet landings (1987-1999). Figure 7. Size frequencies of blue marlin by sex sampled by Venezuelan observers aboard commercial longline vessels (1987-1999). 255

Figure 8. Size frequencies of white marlin by sex sampled from recreational fishing tournaments frequented by U.S. recreational fishermen (1972-1999). 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 (1989-1999). Figure 12. Size frequencies of white marlin by year and sex from fish measured by U.S. observers aboard U.S. commercial longline vessels (1989-1999). 256

Figure 13. Size frequencies of white marlin by sex from port samples of Venezuelan commercial longline and artisanal gillnet landings (1987-1999). Figure 14. Size frequencies of white marlin by sex sampled by Venezuelan observers aboard commercial longline vessels (1987-1999). 257