The shift in marine resource management from a compartmentalized approach of dealing with resources on a species basis to an approach based on management of spatially defined ecosystems requires an ...accurate accounting of energy flow. The flow of energy from primary production through the food web will ultimately limit upper trophic-level fishery yields. In this work, we examine the relationship between yield and several metrics including net primary production, chlorophyll concentration, particle-export ratio, and the ratio of secondary to primary production. We also evaluate the relationship between yield and two additional rate measures that describe the export of energy from the pelagic food web, particle export flux and mesozooplankton productivity. We found primary production is a poor predictor of global fishery yields for a sample of 52 large marine ecosystems. However, chlorophyll concentration, particle-export ratio, and the ratio of secondary to primary production were positively associated with yields. The latter two measures provide greater mechanistic insight into factors controlling fishery production than chlorophyll concentration alone. Particle export flux and mesozooplankton productivity were also significantly related to yield on a global basis. Collectively, our analyses suggest that factors related to the export of energy from pelagic food webs are critical to defining patterns of fishery yields. Such trophic patterns are associated with temperature and latitude and hence greater yields are associated with colder, high latitude ecosystems.
Red drum
Sciaenops ocellatus
are conspicuous members of the ichthyofauna in the coastal zone of the northern Gulf of Mexico and support a popular inshore recreational fishery. Red drum exhibits ...differences in distribution where younger fish (age ≤ 4 years, y) are found primarily in inshore estuarine locations, and older individuals are generally found in offshore locations near Mississippi’s barrier islands. In this study, we evaluate stable isotope values of carbon and nitrogen in muscle tissue of red drum to (1) understand how these vary with the location (bearing) of capture, age (y), and total length (TL, mm), (2) describe how patterns of isotopic niche space and overlap vary with age group, and (3) evaluate if location category (estuary and island) of capture could be predicted using a machine learning approach. We found that patterns of δ
13
C and δ
15
N were variable with bearing, TL, and age. Contrasting patterns of age-specific δ
13
C and δ
15
N values are evident in younger fish (age ≤ 2 y and age 3 to 4 y). Fish from estuaries had lower δ
15
N and lower δ
13
C. To understand age group–specific patterns of isotopic niche space and overlap, we employed a bootstrapping algorithm using SIBER and nicheRover. We observed a minimum overlap of isotopic niche space between the age group comprised of age 1 to 6 y individuals and the age group comprised of age 7 + y individuals. The area of the isotopic niche space was greatest for the age group comprised of age 1 y individuals and for the age group comprised of age 1 to 7 y individuals. To understand if the location category of capture could be predicted, we constructed a random forest model. We found that the variables δ
13
C, δ
15
N, and TL are the most important predictors of the location category of capture. The out-of-bag error rate for the classification was 5%. The movement dynamics of red drum continue to challenge managers charged with the sustainability of the stock. This work highlights the utility of stable isotope analysis for understanding changes in diet and habitat use of red drum and the use of length as a predictive variable for the prediction of location category.
Abstract
We evaluated the effects of multiple bottom-up processes on the relative condition (an index of length-specific weight) of adult Gulf menhaden Brevoortia patronus sampled from the US ...commercial fishery, from 1964 to 2011, in the northern Gulf of Mexico (NGOM). A series of Generalized Additive Models (GAMs) were constructed to examine the influence of Mississippi River Discharge (MRD), El Niño Southern Oscillation (ENSO), wind vector components, sea surface temperature (SST), and chlorophyll a concentration. Relative condition was positively correlated with MRD and highest during weak La Nina-like and moderate El Nino-like conditions. The effect of wind vector components was variable across the NGOM. This observation is likely due to geographic differences in wind-related transport of nutrient rich river plume waters. Relative condition was highest at 24°C and declined rapidly when SST was below 23°C or above 29°C. Relative condition exhibited seasonal variability with increasing condition from August until November, likely caused by seasonal provisioning by individuals for winter spawning. We show that multiple bottom-up processes contribute to contrasts in the condition of Gulf menhaden in the NGOM and these results can be used to predict the response of Gulf menhaden to environmental variability.
Abstract
The haddock (Melanogrammus aeglefinus) stock on Georges Bank in the Northwest Atlantic is characterized by extremely large recruitment events relative to spawning-stock biomass. Recent work ...has indicated that the dynamics of the preceding autumn bloom may have explanatory power to describe these events. In this paper, we examine the hypothesis that autumn phytoplankton dynamics affect the recruitment of haddock, examine the temporal and spatial characteristics of the autumn phytoplankton bloom on Georges Bank, and correlate individual sex-specific condition measurements of haddock made in spring to recruitment patterns. Autumn bloom characteristics vary considerably across Georges Bank with earlier-occurring and larger-integral blooms occurring on the northern flank. On average, autumn blooms start on day 273 (29 September) and persist ∼50 days. There was a significant negative correlation detected between bloom start date and recruitment and a significant positive correlation of bloom integral and recruitment. The survivor ratio loge(R/SSB) was positively and significantly correlated with individual condition of females in spring. The analysis of autumn bloom on Georges Bank provides a predictive index for recruitment strength of haddock and has utility for the assessment of this stock.
Age and growth of early‐life‐stage Atlantic tarpon Megalops atlanticus collected from Mississippi coastal waters in the northcentral Gulf of Mexico (GOM) are described using otolith microstructure ...analysis. Tarpon leptocephali (n = 95, 16.0—27.8 mm standard length, LS) collected from June throughOctober 2013—2018, ranged in age from 22 to 43 days (mean = 30.9 ± 0.5 days). Leptocephalus somatic growth rates ranged 0.46—1.24 mm day−1 (mean = 0.76 ± 0.02 mm day−1), and leptocephalus otolith growth rates ranged 1.78—3.97 μm day−1 (mean = 2.58 ± 0.04 μm day−1). Growth rates were inversely correlated to leptocephalus age, indicating the shrinkage phase associated with leptocephalus metamorphosis. Juvenile tarpon (n = 358, 50—359 mm fork length, LF) were collected from August through December 2007—2018. Juveniles exhibited a positive allometric relationship (adjusted R2 = 0.99, P < 0.001) between length and mass. The age of 100 juveniles (71—277 mm LF) ranged from 76 to 174 days. Juvenile growth rate was estimated as 1.56 ± 0.11 mm day−1. Significant (P < 0.001) linear relationships were found between juvenile age and otolith metrics, including otolith mass (R2 = 0.81) and radius (R2 = 0.68). Evaluation of the backcalculated hatch dates suggests that specimens in the collection hatched from late May through mid‐September with slight peaks during July and August. A Rao's Spacing Test of Uniformity indicates the presence of significant lunar periodicity in leptocephalus hatch dates (n = 95, U = 250.1, P < 0.05), with 50% of the leptocephali hatched within 5 days (before or after) of the full moon. This study fills critical gaps in the scientific knowledge of tarpon and provides estimates of early‐life‐history metrics for an iconic game fish at the northernmost extent of its GOM range.
We used remote sensing chlorophyll a concentration data, spring copepod abundance, and individual fish condition information to understand the annual recruitment variability of two neighboring ...haddock (Melanogrammus aeglefinus) stocks in the Gulf of Maine region. When we considered the full range of recruitment variability, the abundance of the copepods Calanus finmarchicus and Pseudocalanus spp. failed to explain the variation in survivor ratio in either stock. However, when we examined this relationship with subsets of the data, we found that Pseudocalanus spp. appears to have had an effect on survivor ratio. The full range of recruitment variability of the Georges Bank stock was found to correlate with the timing and size of the fall bloom the year before recruitment, which has been termed the parental condition hypothesis, suggesting that the fall bloom affects the condition of spawning adults and thus recruitment. The absence of a correlation between fall bloom and recruitment in the Gulf of Maine stock can be attributed to the difference in fall bloom frequency between the two stock areas. It appears that both parental condition and larval survival affect haddock recruitment; however, the relative impact of these effects depends on the contrasting nature of ecosystem environmental drivers.
The quantification of niche diversity and niche overlap is useful for understanding the impacts of fisheries regulation and management. In this work, we evaluate isotopic niche size and overlap at ...the guild and species levels. Specifically, we analyzed guild‐ and species‐specific isotopic niche space and niche space overlap using the SIBER and nicheROVER packages, and we evaluated length‐specific and geographic contrasts in isotopes for fishes in the north‐central Gulf of Mexico. Stable carbon and nitrogen isotope values (δ13C and δ15N) of fish species were compared among four ecotype‐based fish guilds: estuarine, coastal migratory pelagic (CMP), reef, and large offshore pelagic (LOP). Significant differences in the mean stable isotope values were found among fish guilds. Estuarine guild species exhibited the highest δ13C variability, and Red Drum Sciaenops ocellatus were the most isotopically diverse. Variability of δ13C for CMP fish was comparable to that of some estuarine species, whereas reef and LOP fish were less variable. Fishes within the LOP guild had the largest δ15N range. Reef guild fishes had the smallest isotopic niche space and LOP guild species consistently had the largest, although no species in any guild occupied as large of an isotopic niche space as Red Drum. Distinct and well‐separated isotopic niches were also observed between most estuarine species and CMP species; however, high niche overlap was observed for species within the reef and LOP guilds. We found a positive relationship of δ15N and fish TL for Red Drum, Blackfin Tuna Thunnus atlanticus, and Yellowfin Tuna T. albacares, and we found that δ15N values for Red Drum and Red Snapper Lutjanus campechanus showed distinct isotope differences between geographic areas. This study provides data not only on how species within and without ecotypes interact but also on the variability of their interactions, all of which can inform ecosystem‐based fisheries management models.
The spring phytoplankton bloom on the US Northeast Continental Shelf is a feature of the ecosystem production cycle that varies annually in timing, spatial extent, and magnitude. To quantify this ...variability, we analyzed remotely-sensed ocean color data at two spatial scales, one based on ecologically defined sub-units of the ecosystem (production units) and the other on a regular grid (0.5°). Five units were defined: Gulf of Maine East and West, Georges Bank, and Middle Atlantic Bight North and South. The units averaged 47×103km2 in size. The initiation and termination of the spring bloom were determined using change-point analysis with constraints on what was identified as a bloom based on climatological bloom patterns. A discrete spring bloom was detected in most years over much of the western Gulf of Maine production unit. However, bloom frequency declined in the eastern Gulf of Maine and transitioned to frequencies as low as 50% along the southern flank of the Georges Bank production unit. Detectable spring blooms were episodic in the Middle Atlantic Bight production units. In the western Gulf of Maine, bloom duration was inversely related to bloom start day; thus, early blooms tended to be longer lasting and larger magnitude blooms. We view this as a phenological mismatch between bloom timing and the “top-down” grazing pressure that terminates a bloom. Estimates of secondary production were available from plankton surveys that provided spring indices of zooplankton biovolume. Winter chlorophyll biomass had little effect on spring zooplankton biovolume, whereas spring chlorophyll biomass had mixed effects on biovolume. There was evidence of a “bottom up” response seen on Georges Bank where spring zooplankton biovolume was positively correlated with the concentration of chlorophyll. However, in the western Gulf of Maine, biovolume was uncorrelated with chlorophyll concentration, but was positively correlated with bloom start and negatively correlated with magnitude. This observation is consistent with both a “top-down” mechanism of control of the bloom and a “bottom-up” effect of bloom timing on zooplankton grazing. Our inability to form a consistent model of these relationships across adjacent systems underscores the need for further research.
•Spring blooms are a variable feature of the US Northeast Continental Shelf system.•Bloom duration was inversely related to bloom start date, early blooms last longer.•Bloom initiation was related to the timing of the spring thermal transition.•Spring zooplankton biovolume showed a mixed response to phytoplankton production.•Zooplankton biovolume was alternatively related to spring bloom biomass and timing.
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