Vertical distributions of dolphinfish (Coryphaena hippurus) in the Eastern Pacific Ocean suggest variability in potential associations with floating objects

Abstract

Floating objects play a pivotal role in pelagic ecosystems by serving as shelters, meeting points, cleaning stations, nurseries, and feeding grounds. The abundance of these objects is increasing globally in the form of flotsam, plastics, discarded or lost fishing gear, and fish aggregating devices (FADs) deployed by commercial fisheries. However, it is difficult to measure how often and in what ways fish interact with floating objects in pelagic environments. Dolphinfish (Coryphaena hippurus) is prevalent among the fish species that associate with floating objects, but the extent to which dolphinfish utilize them is unclear. This study applies existing knowledge of FAD-associated dolphinfish diving behavior to identify periods of potential association with floating objects in a remote telemetry dataset of 23 fish with a total of 678 days at liberty spanning two distinct regions within the Eastern Pacific Ocean. Fish inhabiting waters off the western coast of Baja California Sur, Mexico spent significantly more time exhibiting behavior indicative of association with floating objects than those off the coast of Oaxaca, Mexico. When not exhibiting this behavior, dolphinfish in both regions occupied similar vertical habitats, with western Baja fish utilizing more of the water column than Oaxaca fish. Observed regional differences in behavior were coincident with regional differences in size (Oaxaca fish fork lengths ranged from 103 to 118 cm (mean = 110 cm), while Baja fish ranged from 85 to 106 cm (mean = 93 cm)). Although larger fish in the Baja region displayed behavior consistent with smaller Baja fish, future studies should investigate whether the observed regional differences are due to (i) size, (ii) sex, (iii) oceanography, or (iv) availability of floating objects. Dolphinfish are an important mid-trophic level predator and potentially sustainable fishery resource. Understanding their behavior and use of floating objects is of both ecological and economic importance-particularly in the context of expanding international FAD-based fisheries. Our study suggests dolphinfish spend a large amount of their time exhibiting potential floating object associated behavior, and this could influence their population structure and growth.

Fig 1. Seasonal patterns of currents (represented by black arrows) and convergence (represented by color) in the study region.

Boxes denote the spatial extent of the tagging data in each region. The California Current strengthens along Baja in summer bringing kelp into the WBC region; while currents in summer through fall and the rainy season bring logs from the river deltas in Central America to the OAX region in the winter months. Tagged fish were available in WBC in summer and fall; while fish were available in winter and spring in OAX.

Fig 2. Day and night depth histograms for the 3 behavioral modes.

(a) Potential floating-object-associated behavior, (b) Surface-Oriented behavior, and (c) Potential floating-object-unassociated diving behavior. The sample numbers are the total number of diurnal periods that each behavior was observed. White bars denote daytime while black bars denote nighttime distributions.

Fig 3. Distribution and proportion of potential floating-object-associated behavior.

Behavioral changes along tracks in the (a) WBC and (b) OAX regions. (c) Spatial differences in the frequency of behavioral modes. Sample sizes are in brackets for each 5-degree grid (number of 12-hour periods, number of fish).

Fig 4

Size and Region comparison of frequency of (a) pFO-A and (b) pFO-U behaviors.

Fig 5. Principal component analysis of oceanographic conditions.

Colors represent (a) latitude, (b) fish size, and (c) behavior.

Fig 6. Season and fish size cofactors in dataset.

Limitations in sampling (deployment lengths and fishing seasons) have led to larger fish in OAX during the winter to spring months and smaller fish in the WBC region during the summer to fall months.