Understanding the Stock
In the southwest Pacific, striped marlin are a quintessential “shared” stock: a fast‑growing, highly migratory billfish that moves across national waters and the high seas, is caught mainly by longline fleets in the subtropics, and also supports vibrant sport fisheries off eastern Australia and New Zealand. Recent fishery overviews show that most catches in the Western and Central Pacific Convention Area (WCPFC‑CA) come from longline gear in subtropical bands, with hotspots around the Coral and Tasman Seas; total WCPFC‑wide striped marlin catch in 2024 was provisionally about 4,263 t, slightly up on 2023 but still low relative to the 1970s–1990s and broadly stable since the mid‑to‑late 2000s. These patterns frame the southwest Pacific component as a modest but persistent fishery by volume, with outsized social and economic salience for coastal states because it straddles commercial and recreational sectors.
Stock condition
The most recent Bayesian surplus‑production assessment (1952–2022) indicates that Southwest Pacific striped marlin is overfished but not currently undergoing overfishing. When summarized against production‑model reference points, the “recent” period (depletion averaged over 2019–2022; fishing mortality averaged over 2018–2021) gives D/DMSY and F/FMSY both near 0.77, with an estimated 74% probability that depletion is below DMSY and only about 23% probability that overfishing is occurring. Ten‑year projections at status‑quo catches show a continued rebound, with the ensemble median D/DMSY rising above one (≈ 1.32 by 2032), implying recovery toward levels consistent with MSY if recent conditions persist.
While the trajectory is positive, uncertainty in absolute population scale remains a dominant issue. A companion integrated, age‑structured Stock Synthesis (SS3) revision reproduces broad trends but estimates a very low biomass scale and cautions that SB‑based reference points may therefore be overly pessimistic. The SS3 team advises reading its outputs alongside the production model and empirical indicators until scale is better anchored by new data.
Current fishery status
Striped marlin in the assessment domain is taken primarily by longline fleets (both distant‑water and Pacific Island fleets) as a valuable byproduct, with opportunistic targeting in places such as Australia’s East Coast Tuna and Billfish Fishery and a long‑standing recreational fishery in Australia and New Zealand. Effort has increased dramatically over the historical period, from near zero in the early 1950s to roughly 100–150 million hooks per year through the 1970s–1990s, then jumping to about 350 million hooks around 2003 and fluctuating near that level thereafter; total removals have been broadly stable with a slight recent decline.
Importance of the Fishery
Although tuna dominate regional value, longline fisheries remain economically important across the WCPO. In 2024, economic indices for longline fisheries weakened largely because of price declines, which can influence incentives around retention, discarding, and reporting for non‑target species such as striped marlin. Recreationally, striped marlin is culturally and economically significant, with club records in New Zealand dating to the 1920s and substantial sport‑fishing activity in both New Zealand and Australia.
Science
Stock status – latest assessment
The data‑moderate Bayesian surplus‑production model (BSPM) adopts simulation‑based priors for productivity and scale, treats catch and index observation error explicitly, and reports status using D and F relative to MSY. By design it does not generate SB‑based quantities. The 2025 BSPM places the stock below DMSY and below FMSY, shows a pronounced decline to the mid‑2010s followed by a sustained recovery, and projects continued improvement under recent catches (median D/DMSY > 1 within a decade). Its status summaries use “recent” (2019–2022 for depletion; 2018–2021 for F) and “latest” (2022 for depletion; 2021 for F) windows.
In parallel, the revised SS3 assessment (requested by SC20) addresses key diagnostics from 2024 but still infers implausibly low absolute biomass and advises caution with SB‑based reference points until population scale is better resolved. The assessment scientists recommend considering SS3 together with the BSPM and the empirical CPUE series when forming advice.
Scientific advice to managers
Read jointly, the two assessments support maintaining catches at or below recent levels to sustain the observed recovery while uncertainty is reduced. Priority data needs include programs that can resolve population scale and structure—notably Close‑Kin Mark–Recapture (CKMR) to estimate absolute abundance, expanded age–length (otolith) sampling to refine growth and natural mortality, and genetic work to clarify stock connectivity. Embedding the stock within a harvest‑strategy framework tested via Management Strategy Evaluation (MSE) is a clear next step given the uncertainty profile.
How does the WCPFC manage the Southwest Pacific striped marlin fishery?
Current management framework
WCPFC CMM 2006‑04 limits each CCM’s number of vessels fishing for striped marlin south of 15°S to the level in any one year from 2000–2004, with reporting requirements on vessel numbers and catches, explicit recognition of SIDS/coastal state development rights, and monitoring via the Technical and Compliance Committee (TCC). The measure’s preamble notes Scientific Committee advice to avoid increases in fishing mortality while status estimates are uncertain. Some coastal states with commercial landing moratoria are exempt.
Management issues
Because a cap on vessel numbers is only a proxy for fishing mortality, changes in fishing efficiency, deployment patterns, and targeting can allow mortality to rise without violating the letter of the measure. That disconnect is relevant in mixed longline fisheries where shifts in targeting (e.g., responding to tuna prices) can alter striped marlin catches even with stable fleet numbers. The assessments also emphasize data limitations, including uncertainty in early catches, possible under‑reporting, divergence among CPUE indices, and unresolved stock structure with indications of connectivity beyond the model domain, all of which have an impact on the reliability of status metrics.
Improving Future Management
Science and management both point to a harvest‑strategy/MSE pathway with explicit objectives, performance metrics, and tested control rules (effort or catch‑based, potentially complemented by spatial/temporal measures and handling standards). Doing so requires stronger monitoring (e.g., observer coverage and standardized CPUE workflows), targeted research to pin down absolute scale and life‑history parameters (CKMR, otolith aging, genetics), and, in due course, a review of CMM 2006‑04 so that controls act more directly on mortality and align with agreed reference points.
Summary
The best current reading from the BSPM is that Southwest Pacific striped marlin is overfished, not undergoing overfishing, and recovering under recent catch levels; projections suggest median depletion above DMSY within roughly a decade if recent catches continue. However, the SS3 assessment revision highlights substantial population‑scale uncertainty, implying that SB‑based reference points are likely too pessimistic and should be interpreted with care until scale is better established. Management is presently based in CMM 2006‑04 (vessel caps south of 15°S), but both assessments support transitioning to a harvest‑strategy framework backed by improved monitoring and focused research to reduce uncertainty and align controls more tightly with fishing mortality.