Taxonomic composition, abundance and habitat associations of squid paralarvae in the northern Gulf of Mexico

Highlights

• Upwelling areas and river discharge create productive habitat for squid paralarvae.

• Preference for summer hatching in the northern Gulf of Mexico.

• Main paralarvae families are Ommastrephidae, Enoploteuthidae and Onychoteuthidae.

• Squid paralarvae exhibit diel vertical migration patterns.

Abstract

Summer plankton surveys were conducted in 2015–2017 to characterize the distribution and abundance of squid paralarvae in epipelagic waters of the northern Gulf of Mexico (Gulf). Paralarvae present at stations sampled were from 12 families, with the most abundant being Ommastrephidae (flying squids), Enoploteuthidae (armed squids) and Onychoteuthidae (hooked squids). Mean density and percent frequency of occurrence for squid paralarvae across all surveys was 8.8 paralarvae 1000 m⁻³ and 76%, respectively. Julian day, salinity, sea surface height (SSH) and time of day were identified as influential environmental variables in generalized additive models (GAMs). Paralarval densities peaked during early morning and late evening sampling times, which is in accord with diel vertical migration patterns. Densities increased in early July, in areas with low SSH and lower salinity (28–35 psu), indicating upwelling areas and proximity to inflow from the Mississippi River represent productive early life habitats for squid. Results suggest that oceanic squid spawn in the northern Gulf in the summer exploiting the position of mesoscale oceanographic features and the extension of freshwater discharges from the Mississippi River.

Introduction

Pelagic cephalopods, primarily squids, are dominant prey for several marine organisms, including migratory pelagic fishes (i.e. billfishes, tunas, and sharks), sea birds, and marine mammals (Clarke, 1996; Logan et al., 2013; Rodhouse et al., 2013; Staudinger et al., 2013). Squids are voracious predators as well and are essential components in open-ocean food webs, transforming biomass of micronekton into high-energy food for large predators (Clarke, 1996). It has been estimated that the yearly consumption of squids by sperm whales alone is greater than the total world catch of all marine and freshwater organisms combined (Clarke, 1977, as cited in Hoving et al., 2014). Squids occupy a large range of trophic levels, even exhibiting significant top-down control on their prey; however, as an important prey to large pelagic predators, open-ocean squids are constrained to a bottom-up control on their predators and can be regarded as keystone species in pelagic ecosystems (Coll et al., 2013). In fact, squid populations are increasing as predation and competition is reduced due to overexploited fish stocks (Caddy and Rodhouse, 1998).

Squids grow rapidly, have short lifespans (usually <2 years), and are semelparous, resulting in a fast life-history strategy (O'Dor and Webber, 1986). While this life strategy allows squids to be opportunists and take advantage of favorable environmental conditions, sudden deterioration of the environment or food sources can cause rapid declines in abundance and result in pronounced interannual variation in population sizes (Rodhouse et al., 2014). Despite these fluctuations, recent studies suggest that cephalopod populations are steadily increasing over time due to oceanic environmental changes (Doubleday et al., 2016). Still, cephalopod population dynamics are poorly understood and difficult to forecast due to their adaptability to changing conditions and opportunistic life history traits (Rodhouse et al., 2014; Doubleday et al., 2016). An improved understanding of squid abundance and habitat associations based on environmental factors during the pelagic paralarval stage is critical to population modeling efforts because recruitment variability is commonly linked to early life events (Cowen et al., 2000).

Little is known about the early life ecology of pelagic squids in the northern Gulf of Mexico (Gulf), which is a region that maintains some of the most productive fisheries in the United States (Chesney et al., 2000). The influences of the Mississippi River and mesoscale features, such as the Loop Current and associated eddies, provide nutrient rich habitats that increase primary and secondary production (Govoni et al., 1989; Biggs, 1992). Further, the northern Gulf is also recognized as an important spawning and nursery habitat for several large pelagic predators that are known to feed heavily on cephalopods, including billfishes, tunas, and cetaceans (Rooker et al., 2012; Ruiz-Cooley et al., 2012; Kitchens and Rooker, 2014; Judkins et al., 2013; Cornic et al., 2018). The purpose of the present study is to use a generalized additive modeling framework to characterize early life habitats of squids and to identify influential environmental parameters that affect their relative abundance and distribution in this basin.