[Paper Review] Tuna and billfish larval distributions in a warming ocean

Hirotaka Ijima, Marko Jusup|arXiv (Cornell University)|Apr 19, 2023

Marine and fisheries researchEnvironmental Science3 citations

TL;DR

This study uses a geostatistical species-distribution model trained on the largest Pacific Ocean tuna and billfish larval dataset (1960–1985) to map basin-wide seasonal larval distributions, project impacts of ocean warming, and identify key biogeochemical drivers. It reveals species-specific responses to warming, with most tuna species facing negative impacts and Pacific bluefin tuna benefiting from increased SST in its larval habitat.

ABSTRACT

Tuna and billfish are charismatic pelagic fishes attracting considerable scientific attention due to their ecophysiological and socioeconomic importance. However, the knowledge of their basin-wide spawning and larval habitats, especially in a warming ocean, is limited. This knowledge gap undermines effective fishery management by introducing recruitment uncertainty, which makes population dynamics unpredictable. We fill the gap with a parsimonious geostatistical species-distribution model trained on the largest available dataset on tuna and billfish larvae in the Pacific Ocean. The model reveals (i) the basin-wide seasonal larval distributions over the reference period 1960-85, (ii) the expected impact of ongoing ocean warming on these distributions, and (iii) the biogeochemical factors, such as pH, phosphate concentration, and sea-surface height, that shape the larval habitat. Our findings make a quantum leap in understanding the ecophysiology of tuna and billfish, providing valuable information for future conservation efforts.

Motivation & Objective

• Address the critical knowledge gap in basin-wide spawning and larval habitat dynamics of tuna and billfish, especially under ocean warming.
• Reconstruct seasonal larval distributions across the Pacific Ocean for the reference period 1960–1985.
• Project the expected impact of ongoing ocean warming on larval habitat suitability and distribution.
• Identify biogeochemical factors—such as pH, phosphate, and sea-surface height—that shape larval habitat conditions.

Proposed method

• Employed a parsimonious geostatistical species-distribution model using larval survey data from 1960 to 1985 across the Pacific Ocean.
• Trained the model on species identity, geolocation, sampling effort (m³·min⁻¹), and sea-surface temperature (SST) from 24 species or multi-species groups, focusing on nine confidently identified species.
• Used latent predictors and spatial-temporal correlation structures (via Matérn covariance functions) to model unobserved environmental gradients and autocorrelation in larval abundance.
• Applied Tweedie distribution to model overdispersed count data of larval densities, accounting for zero-inflation and non-normality.
• Conducted rigorous model diagnostics using randomized quantile residuals (RQRs) to detect overfitting, misspecification, and distributional issues.
• Validated model robustness using artificial datasets and sensitivity analyses, including omission of single-species data or alternative functional forms (e.g., parabolas vs. splines for temperature response).

Experimental results

Research questions

• RQ1 What are the basin-wide seasonal patterns of tuna and billfish larval distributions in the Pacific Ocean during the reference period 1960–1985?
• RQ2 How will ongoing ocean warming alter the spatial and seasonal distribution of tuna and billfish larval habitats?
• RQ3 Which biogeochemical variables—such as pH, phosphate concentration, and sea-surface height—are most predictive of larval habitat suitability?
• RQ4 How do different species respond to warming, and what explains the divergent trends (e.g., negative vs. positive aggregate effects) among species?

Key findings

• Yellowfin tuna, skipjack tuna, and blue marlin larvae exhibit broad equatorial distributions with seasonal northward (boreal summer) and southward (austral summer) shifts.
• Bigeye tuna, albacore tuna, swordfish, striped marlin, and sailfish larvae occupy patchy, seasonally variable habitats, with presence strongly correlated with warm months away from the equator.
• Pacific bluefin tuna larvae are exclusively found in the northwestern Pacific during boreal spring and summer, indicating a highly localized and seasonal spawning distribution.
• A negative aggregate effect of ocean warming is projected for four tuna species (bigeye, yellowfin, albacore, skipjack), indicating reduced larval habitat suitability.
• Swordfish show minimal change in larval habitat under warming, suggesting relative resilience to projected SST increases.
• Pacific bluefin tuna and three billfish species (striped marlin, blue marlin, sailfish) are projected to experience a positive aggregate effect, with increased larval habitat suitability due to warming in their core regions.