Market squid (Doryteuthis opalescens) are ecologically and economically important to the California Current Ecosystem, but populations undergo dramatic fluctuations that greatly affect food web dynamics and fishing communities. These population fluctuations are broadly attributed to 5–7‐years trends that can affect the oceanography across 1,000 km areas; however, monthly patterns over kilometer scales remain elusive. To investigate the population dynamics of market squid, we analysed the density and distribution of paralarvae in coastal waters from San Diego to Half Moon Bay, California, from 2011 to 2016. Warming local ocean conditions and a strong El Niño event drove a dramatic decline in relative paralarval abundance during the study period. Paralarval abundance was high during cool and productive La Niña conditions from 2011 to 2013, and extraordinarily low during warm and eutrophic El Niño conditions from 2015 to 2016 over the traditional spawning grounds in Southern and Central California. Market squid spawned earlier in the season and shifted northward during the transition from cool to warm ocean conditions. We used a general additive model to assess the variability in paralarval density and found that sea surface temperature (SST), zooplankton displacement volume, the log of surface chlorophyll‐a, and spatial and temporal predictor variables explained >40% of the deviance (adjusted r2 of .29). Greatest paralarval densities were associated with cool SST, moderate zooplankton concentrations and low chlorophyll‐a concentrations. In this paper we explore yearly and monthly trends in nearshore spawning for an economically important squid species and identify the major environmental influences that control their population variability.
To test the hypothesis that prey partitioning contributes to community stability in flyingfish, the gut contents of 359 flyingfish specimens (representing five genera and eight species within Exocoetidae and Hemiramphidae) were collected at 50 dip-net stations during hour-long night-time fishing in oceanic waters of the eastern tropical Pacific Ocean between August and November 2007. Analyses using multidimensional scaling, and analysis of similarity revealed significant dietary differences among species, and similarity percentages tests helped identify the specific prey taxa responsible for these differences. Six species specialized on copepods (58·3-96·9% by number), but targeted different families. Specifically, the barbel flyingfish Exocoetus monocirrhus (n = 205) focused on euchaetids (51·6%), the banded flyingfish Hirundichthys marginatus (n = 24) fed on pontellids (21·8%), while the tropical two-wing flyingfish Exocoetus volitans (n = 11) and the bigwing halfbeak Oxyporhamphus micropterus (n = 34) ingested calanoids (54·6 and 17·0%). In contrast, the whitetip flyingfish Cheilopogon xenopterus (n = 73) and the mirrorwing flyingfish Hirundichthys speculiger (n = 4) had generalized diets comprising similar proportions of amphipod, copepod, mollusc and larval fish prey. Distinct differences in mean fullness, highly digested material, per cent empty guts and mean numbers of prey per gut were also synthesized, and uncovered a pattern of asynchronous feeding. Altogether, these findings provide valuable descriptive data on the diets of an understudied group of epipelagic teleosts, and, by extension, suggest that prey partitioning (taxa and feeding times) may influence flyingfish feeding ecology by reducing interspecific competition.
Abstract-We examined the stomach contents of 3 vertically migrating myctophid fish species from the eastern tropical Pacific (ETP) Ocean and used a classification tree to examine the influence of spatial, biological, and oceanographic predictor variables on diet. Myctophum nitidulum (n=299), Symbolophorus reversus (n=199), and Gonichthys tenuiculus, (n=82) were collected with dip nets from surface waters, and prey taxa were quantified from bongo net tows from August through November 2006. A classification tree produced splits with longitude and sea surface salinity (SSS), thereby separating 3 geographically and oceanographically distinct regions of the ETP (offshore, nearshore, and intermediate), where diet was similar among the 3 species. Myctophids consumed, primarily, ostracods offshore (76.4% mean percentage by number [MN i ]), euphausiids nearshore (45.0%), and copepods (66.6%) in the intermediate region. The offshore region was characterized by a greater abundance of ostracods in the zooplankton community (17.5% by number) and within a deep mixed-layer depth (MLD) (mean 52.6 m, max 93.0 m). SSS was low in the nearshore region (<32.9 psu) and the MLD was shallow. The intermediate region represented a transition zone between the oceanographic condition of the offshore and nearshore regions. Our results indicate that these 3 myctophid species share a similar regional diet that is strongly influenced by longitude, ostracod availability, SSS, and MLD.The Myctophidae (lanternfishes) comprise a family of fishes whose members are both extremely abundant and distributed throughout the world's oceans (Gjosaeter and Kawaguchi, 1980; Irigoien et al., 2014). Species making up this family of fish serve roles as both important predators (Pakhomov et al., 1996) and prey (Naito et al., 2013); furthermore myctophids transfer energy from lower to higher trophic levels in food webs (Brodeur et al., 1999). Myctophids are also influential in the transfer of carbon to the deep sea because they feed in surface waters and return to the mesopelagic zone (Davison et al., 2013). The family is speciose, with as many as 250 species in 33 genera (Catul et al., 2011). In some instances, as many as 50 species can be found in close proximity, simultaneously feeding on similar prey (Hopkins and Gartner, 1992).Resource partitioning, broadly defined as differences in resource use among co-occurring species (Schoener, 1974), has been used to explain how diverse myctophid assemblages can co-occur without competitively excluding one another (Hopkins and Gartner, 1992). Myctophids have been shown to partition resources by size (myctophid size) (Shreeve et al., 2009;Saunders et al., 2015), migration depth, and prey type (Hopkins and Gartner, 1992; Pepin, 2013 (Cherel et al., 2010;Shreeve et al., 2009), and North Atlantic (Pusch et al., 2004). Conversely, generalist behavior has been described by Kinzer and Schulz (1985), who found that 7 myctophid species in the equatorial Atlantic fed opportunistically on similar calanoid copepods. Pakhomov et ...
The California market squid is ecologically and economically important to the California Current Ecosystem and coastal communities. However, the population undergoes periodic and large-scale fluctuations on the order of magnitudes, largely as a result of warm ocean temperature and reduced ocean productivity related to cycles in El Niño Southern Oscillation. These fluctuations can lead to cascading trophic effects on the ecosystem, market uncertainty, and substantial revenue losses for fishing communities. In order to investigate the role that spawning behavior may play in mitigating the detrimental environmental effects on the population as well as allowing the species to capitalize during favorable ocean conditions, surveys for market squid paralarvae were conducted three times per hatching season from 2012 to 2019. The specific objectives were to (a) assess whether there was a large-scale synchronous peak in paralarval abundance each season, (b) investigate whether market squid paralarvae were found at certain locations in similar abundances throughout the duration of a hatching season, and (c) to understand the influence of oceanographic parameters, such as zooplankton availability, sea surface temperature, and chl-a on paralarvae abundance during each hatching season. Adaptable spawning strategies that varied over time were observed. Market squid displayed large-scale synchronous spawning during cool and productive oceanographic conditions; protracted spawning was observed during warm and oligotrophic conditions. No overall site fidelity was observed across all effort, but during two seasons where habitat compression was possible, market squid consistently spawned in certain areas. Generalized additive models were used to explore the effects of a small set of oceanographic variables on paralarval density on a seasonal basis. Sea surface temperature and geographic variables were generally the most important variables. These findings indicate that market squid spawning behaviors and habitat are adaptable and dependent on oceanography and population density.
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