IATTC Fishery Status Report 8 - Comisión Interamericana del Atún ...

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I. STRIPED MARLIN
This section presents information on the assessment of the stock of striped marlin in the eastern Pacific Ocean (EPO) north of 5°S
(northern EPO), as well as general information on striped marlin. Data on catches (Figure I-1) made by longline and recreational
fisheries were updated to 9 September 2010.
Striped marlin occur throughout the Pacific Ocean between about 45°N and 45°S. They are caught mostly by the longline fisheries
of Far East and Western Hemisphere nations and by recreational fisheries. Lesser amounts are caught by gillnet and other fisheries.
During recent years, the greatest commercial catches in the EPO have been taken by the fisheries of Chinese Taipei, French
Polynesia, and Japan.
The principal recreational fishery for striped marlin in the EPO is that of Mexico, where commercial fishing for marlin and other
billfish species is prohibited. These species are reserved for recreational fisheries within a zone extending up to 50 miles from the
Pacific coast of Mexico. Most of the striped marlin caught in these fisheries are released, but some are landed, usually when a
fisherman decides to take his catch home or when a fish dies during the capture process. Another category of landings occurs in
catch-and-release recreational fisheries; these landings are the unobserved, or cryptic, mortalities that occur when a fish dies
following its release. Results of tagging experiments using pop-up satellite tags suggest a mortality rate of about 25 percent for fish
released in the recreational fishery.
In the case of the recreational fisheries of Mexico, the proportion of the catch that is released has increased over time. The recent
release rate reported by the fishery is 93 percent, while the comparative rate from scientific sampling is 75 percent. The lowest
release rate observed by sampling is about 72 percent, observed in 1999, the first year of sampling. Therefore, lacking data on
release rates prior to 1999, it was assumed that the release rate for catches made during 1990-1998 was 70 percent. There are no data
available on total catch or landings by the recreational fishery prior to 1990.
Information on the movements of striped marlin is limited. Fish tagged with conventional dart tags and released off the tip of the
Baja California peninsula have generally been recaptured near where they were tagged, but some have been recaptured around the
Revillagigedo Islands, a few around Hawaii, and one near Norfolk Island. Tagging studies of striped marlin in the Pacific conducted
using pop-up satellite tags indicated that there is essentially no mixing of tagged fish among tagging areas, and that striped marlin
maintain site fidelity.
Significant effort has been devoted to understanding the stock structure of striped marlin in the Pacific Ocean. It has been clear for some
years that there are a number of stocks, and the recent results of analyses of fisheries and genetic data indicate that the northern EPO is
home to a single stock, though there may be a seasonal low-level presence of juveniles from a more westerly Hawaii/Japan stock.
This assessment is the first following stock-structure research that confirmed the presence and general distribution of this northern
EPO stock. The base case is an age-structured assessment with catch in biomass (t) conducted using Stock Synthesis (Version
3.10.b), which allows integration of multiple categories of data from fisheries, e.g. catch, effort, and sampled distributions of the
catch by size or age, as well as ancillary data which might affect population parameters and dynamics.
This assessment incorporates data on total landings by fishery, catch rates (CPUE: catch-per-unit-effort) from longline fisheries of
Japan, and length measurements of fish taken by the longline and purse-seine fisheries of the northern EPO. Data on growth and age
and weight at length were also used to convert lengths of fish to estimates of weight and age. The age selectivity and retention of
each of the fisheries in the model for which length-frequency data are available is estimated as part of the fitting process, providing
an indicator of the portion of the population exploited by age, gear, and area. The northern EPO was divided into three subareas for
the assessment (Figure I-1), and a standardized CPUE, which provides an index of abundance, was determined for each, using data
from Japanese longline fisheries. Six fisheries were defined in the model: F1, F2, and F3 are the longline fisheries of Japan in areas
1, 2, and 3; F4 is the other longline fisheries of the EPO; F5 is the recreational fishery of Mexico; and F6 is the EPO purse-seine
fishery. Total catch was compiled for each of these fisheries (Figure I-2). When fitted, the model provides estimates of the
selectivity of each of the fisheries based on the size- or age-frequency data. The estimated age selectivities for longline fisheries and
the purse-seine fishery are shown in Figure I-3. Since detailed size-frequency data (the model incorporated length-frequency data in
2-cm intervals) were not available for the recreational fishery, in the base case the selectivity for that fishery was estimated using the
selectivity of the longline fishery in Area 3.
Longline fisheries expanded into the EPO beginning in the mid-1950s, and they extended throughout the region by the late-1960s.
Except for a few years in the late-1960s to early 1970s in the northern EPO, these fisheries did not target billfish. The shifting