Aerial survey as a tool for understanding bigeye scad (Selar crumenophthalmus) dynamics around the island of O'ahu, Hawai’i
Introduction
Fishery-dependent catch and effort data are commonly utilized to generate abundance indices for fish stocks based on the assumption that catch per unit effort (CPUE) is proportional to abundance. However, it has long been argued CPUE may not accurately reflect true changes in abundance (Beverton and Holt, 1957), and a more recent analysis supports this argument (Harley et al., 2001). Hilborn and Walters (1992) discuss an alternative relationship between CPUE and abundance called hyperstability, where CPUE remains high as abundance decreases. Pelagic schooling species are especially prone to hyperstability; fewer schools are present as abundance declines, yet school densities remain preserved, and therefore fishermen can harvest consistent catches (Brierley and Cox, 2015). Basing fishery management decisions on the false assumption that catch is proportional to abundance can contribute to declining fish stocks (Rose and Kulka, 1999).
Bigeye scads (Selar crumenophthalmus) are a schooling coastal pelagic species and have a circumtropical distribution throughout the Atlantic, Indian, and Pacific Oceans, including the warm coastal waters of all the Hawaiian Islands. In Hawai’i, bigeye scads are commonly referred to as "akule" when their total length exceeds 22 cm; shorter individuals are called "halalū" (Iwai et al., 1996). We use this convention throughout this paper and use the term akule only when referring exclusively to large bigeye scad.
Bigeye scad has a high growth rate (K = 0.21/month) signifying high productivity (Kawamoto, 1973) and reaches sexual maturity at a standard length of about 20 cm (Clarke and Privitera, 1995). Captive males and females reach maturity at a fork length of 19 cm and 25 cm, respectively (Iwai et al., 1996). Weng and Sibert (2000) estimated sexual maturity occurs after 7 months based on a von-Bertalanffy growth curve fit to length data produced from tag-recapture experiments by Kawamoto (1973). The months of April - October are the main spawning period (Clarke and Privitera, 1995). Recently recruited schools first appear in July and persist through December, suggesting a 4-month (April–July) ichthyoplankton growing phase (Kawamoto, 1973). Bigeye scad raised in captivity also spawn in accordance to these natural patterns during their first year. After the initial year, the broodstock spawned repeatedly (5–10 times) throughout each of the following two years (Iwai et al., 1996). Captive bigeye scads grew to a mean total length of 13.24 cm 141 days post hatch (Welch et al., 2013). The estimated annual mortality fraction for bigeye scad within Hawai’i is 99.3 %, though these fish may survive over two years (Kawamoto, 1973).
Bigeye scad represents the largest proportion of total landings by weight among species listed in the Fishery Ecosystem Action Plan (Western Pacific Regional Fisheries Management Council (WPRFMC, 2009) for the Hawaiian Archipelago. The most effective method used to commercially harvest bigeye scad utilizes surround net fishing gear. Gill nets are most often used and less labor intensive than deploying fence (bag) nets which require SCUBA divers to close the nets (See Kazama 1997 for fishing techniques). These operations are often performed in coordination with spotter planes because the planes are far more efficient in locating schools of fish than are boats. Each morning, bigeye scad schools form in the coastal areas surrounding O'ahu. The spotter plane relays the location and estimated size of these schools to the fishermen who rely on the pilot's accurate biomass estimates to determine the most advantageous school to pursue. Fishermen often target a school in the morning that was spotted the previous day. The same fish aggregation might also be fished multiple times, over many days, as it repeatedly reforms. These accounts are supported by a tag and release study that found little movement among individual bigeye scad around O'ahu and no recaptures on neighboring islands (Kawamoto, 1973).
As specified by the Magnuson-Stevens Fishery Conservation Reauthorization Act of 2006, fish species listed in Fishery Ecosystem Action Plans (FEP) are required to have reference points based on the best available science (MSRA, 2007). A stock assessment for bigeye scad was previously conducted in 2000, using historical catch and effort information from the state of Hawai'i commercial reporting database (Weng and Sibert, 2000), and therefore bigeye scad reference points were calculated using fishery-dependent data. Weng and Sibert (2000) applied a surplus production model (Schaefer, 1954) and determined bigeye scad has undergone light to moderate exploitation suggesting that yearly catch fluctuations are more dependent on social pressures dictating effort rather than population abundance. However, should assumptions about catchability being proportional to bigeye scad abundance not hold, this conclusion may be unjustified. Despite being heavily fished, bigeye scad was removed from the FEP as a management unit species and placed into an ecosystem component categorization in 2018 (National Marine Fisheries Service (NMFS, 2019). Although the stock status of bigeye scad is not currently required to be assessed relative to established reference points, they must still be monitored. The potential for hyperstability warrants exploring whether fishery-independent trends in abundance differ from current sources of information, and if monitoring of populations in the future should be conducted using fishery-independent approaches, or continue based on commercial catch and effort information.
Aerial surveys provide a method to estimate abundance of marine species independent of a fishery and thus provide a time series of data to compare indices of abundance with fishery-dependent sources of information. The use of aerial spotting planes has provided a means for direct observations of many marine species found at or near the ocean surface. This method of surveillance has been largely utilized in scientific surveys for estimating population sizes and distributions of marine animals spanning many taxonomic groups, such as turtles (Marsh and Saalfeld, 1989), dolphins (Slooten et al., 2004), dugongs (Pollock et al., 2006), sharks (Cliff et al., 2007), tunas (Basson and Farley, 2014), and coastal pelagic species (Lynn et al., 2014).
This study describes the results from a fishery-independent, qualitative aerial survey of bigeye scad apparent abundance, defined “…as the abundance as affected by availability, or the absolute number of fish accessible to a fishery (Marr, 1951),” around the island of O'ahu in the Hawaiian Islands. The first objective of our study was to compare the index of apparent abundance from a fishery-independent aerial survey to a fishery-dependent index based on catch records in the commercial fishery and assess the potential for bigeye scad hyperstability. We analyzed the index across different spatial and temporal scales to determine differences due to scale and to improve future sampling design. The second objective was to describe spatial and temporal patterns in bigeye scad dynamics and reveal local distribution, general movement, and timing of spawning around O'ahu. This information can be used by managers to inform best approaches for future monitoring of bigeye scad relative apparent abundance and better understand movement and recruitment dynamics within the Hawaiian Islands.
Section snippets
Materials and methods
Bigeye scad represent an ideal test species because of their predictable near-shore schooling behavior each morning around the island of O'ahu. A spotter pilot was chartered to collect data on the number and biomass of bigeye scad schools around O'ahu over the course of 12 months from November 2015 to October 2016 (Pacific Islands Fisheries Science Center (PIFSC, 2019). A single pilot was used to limit possible bias introduced by varying spotting ability (Lo et al., 1992). Ten flights were
Aerial survey
The pilot spotted a total of 854 schools of bigeye scad surrounding the island of O'ahu during the survey period. Estimated school sizes ranged from 500 to 60,000 lbs, with a mean size of 7,702 (SE = 230) lbs. During the beginning season (November–January), 27 flights resulted in a mean of 5.4 (SE = 0.38) schools and 48,374 (SE = 5,035) lbs spotted per trip. During the middle season (February–April), 30 flights resulted in a mean of 6.1 (SE = 0.57) schools and 52,767 (SE = 6,701) lbs spotted
Discussion
The use of fishery-independent abundance estimates for bigeye scad offers a comparable methodology to abundance estimates derived from commercial catch and effort data, providing an additional tool to help decipher possible trends in abundance and identify biases and limitations within either of the two approaches. Barnes et al. (1992) suggested that an integrated approach for stock assessment combining data from fishery-independent and fishery-dependent sources will best meet future management
CRediT authorship contribution statement
John Wiley: Conceptualization, Methodology, Investigation, Formal analysis, Writing - original draft. Marlowe Sabater: Conceptualization, Supervision, Project administration. Brian Langseth: Conceptualization, Formal analysis, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Funding for this project was provided by the Western Pacific Regional Fishery Management Council through the United States Department of Commerce, National Oceanic and Atmospheric Administration Cooperative Research Fund grant number NA15NMF4410008. This work would not have been possible without the enormous effort from the pilot Simeon Ivanov and the fishing operations of Kaipo Miller and Island Wide Fishing. Mahalo for all your help and letting me tag along and learn from your extensive
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