Ichthyoplankton Surveys Research Articles

Pelagic habitat and offspring survival in the eastern stock of Atlantic bluefin tuna

Abstract In this manuscript, we test how an understanding of geographical variation in larval fitness in relation to temperature and habitat use could be a useful method to improve our understanding of recruitment and develop better indices of annual recruitment. On the basis of the assumption that growth and survival of tuna larvae are influenced by temperature, we have developed a potential larval survival index for Atlantic bluefin tuna (Thunnus thynnus) by combining empirical data from egg and larval rearing experiments with temperature data from hydrodynamic models. The experiments were designed to test the full range of temperature variability that bluefin larvae would experience in the field and provide a mechanistic understanding of the processes driving egg and larval survival. We then developed a biological model using the temperature-related growth expressions and a size-dependent survival function for the larvae. The biological model was applied to a time-series of spatially explicit temperature data for the western Mediterranean from the Strait of Gibraltar to 6°E, which includes the major recognized bluefin tuna eastern stock spawning area, the Balearic Sea. Our results show that areas with high probabilities of larval survival coincide with those that would be considered as optimal based on other data sources (ichthyoplankton surveys, spawning female locations from commercial fisheries data, and adult tracking data). However, evidence of spawning has been found in areas with suboptimal thermal habitats, as predicted by the model, which we discuss regarding sampling effort and salinity fronts. There was a good match between the survival index and recruitment indices from standardized CPUE fisheries data. These results have implications for our understanding of the recruitment process of the eastern stock of Atlantic bluefin tuna, since they suggest that the combined effects of temporal and spatial variability of the environment drive recruitment success, which has important implications for the management of the species.

Integrating reproductive ecology, early life dynamics and mesoscale oceanography to improve albacore tuna assessment in the Western Mediterranean

Albacore tuna is a pelagic top predator of high ecological and commercial relevance worldwide. Despite the importance of this species, there is only limited knowledge of its life cycle, reproductive dynamics and early life ecology. In the Mediterranean, where the assessment is based on data-poor methods, the status of the albacore population remains uncertain. Here we combine information from commercial fisheries, ichthyoplankton surveys and oceanographic data in the Western Mediterranean Sea to identify the geographical location of the main spawning areas and larval habitats. We applied this information to develop a habitat-corrected larval abundance index providing information on early life dynamics. The larval index was compared with fisheries-dependent indices used for the stock assessment of the Mediterranean albacore. The results allowed the identification of the main spawning grounds, located around the Balearic Sea, and the role of oceanographic conditions when studying larval abundances. The larval abundance index shows a significant negative trend during the last decade, indicating a potential reduction in the spawning stock biomass, which is consistent with the trend observed from the fisheries-dependent indices. We discuss how the systematic integration of information from ichthyoplankton surveys and hydrography to improve assessment is an example of operational fisheries oceanography in practice.

Effect of temperature on Flathead Sole (Hippoglossoides elassodon) spawning in the southeastern Bering Sea during warm and cold years

Between 2001 and 2013 the southeastern Bering Sea alternated between multi-year periods of relatively warm (2001–2005) and relatively cold (2007–2013) years. Adult Flathead Sole Hippoglossoides elassodon in the southeastern Bering Sea shift their spatial distribution pattern in relation to bottom water temperature indicating the importance of the thermal environment in determining fish habitat. The objective of this study was to infer Flathead Sole spawning area in the southeastern Bering Sea in May from ichthyoplankton surveys of their eggs using generalized additive models to assess how warm (2002, 2003, and 2005) and cold (2008, 2009, 2010, and 2012) years affected the location of their spawning site. Three mechanisms through which temperature could affect egg distribution were investigated: 1) additively, assuming a similar effect of temperature across the spatial domain, 2) as a spatially variable term, assuming that the effect of temperature changes across locations, and 3) as a threshold effect on the spatial distribution of eggs, assuming the distribution of eggs changes abruptly in relation to the annual average temperature. The model including temperature as a spatially variable effect had the smallest AIC score and therefore was chosen as the best fit. This model showed that the effect of temperature on egg density varied across the study area and indicated a northeastward expansion of the spawning site as temperature increased. The area of expansion may correspond with the outer reach of a cold-water mass (called the cold pool) that extends in the Bering Sea middle domain during late spring and summer, and forms as a result of the previous winter ice coverage. Flathead Sole are known to avoid temperatures colder than 2 °C that is also used as the boundary for the cold pool, thus spawning fish may have moved relative to the spatial extent of the cold pool.

First record of the non-indigenous jellyfish Blackfordia virginica (Mayer, 1910) in the Baltic Sea

Marine invasions are of increasing concern for biodiversity conservation worldwide. Gelatinous macrozooplankton contain members, which have become globally invasive, for example the ctenophore Mnemiopsis leidyi or the hydromedusae Blackfordia virginica. B. virginica is characterised by a large salinity tolerance, with a brackish-water habitat preference, and by a metagenic life history strategy with an alternation between sexually reproducing planktonic medusae and asexually reproducing benthic polyps to complete the life cycle. In this study we analysed 8 years of ichthyoplankton survey data (2010–2017) from the Kiel Canal and 14 ichthyoplankton summer surveys in the central Baltic Sea (2008–2017). We report the first presence of B. virginica in northern Europe, namely from the southwestern Baltic Sea and the Kiel Canal. In the Kiel Canal, B. virginica was first sporadically sighted in 2014 and 2015 and has developed persistent populations since summer 2016. Changes in size-frequency distributions during summer 2016 indicate active recruitment in the Kiel Canal at salinities between 7 and 13 and temperatures > 14 °C. Close vicinity to and direct connection with the southwestern Baltic Sea, where B. virginica was observed during 2017, indicate that the Baltic Sea and other brackish-water habitats of Northern Europe are at risk for colonisation of this non-indigenous species. Our results highlight that monitoring activities should consider gelatinous macrozooplankton for standard assessments to allow for the detection of non-indigenous species at an early stage of their colonisation.

Spawning patterns provide further evidence for multiple stocks of sardine (Sardinops sagax) off eastern Australia

AbstractPrevious studies have suggested that sardine, Sardinops sagax, off eastern Australia spawns across its entire range when habitat conditions are suitable. However, recent studies have suggested that separate sub‐populations and spawning groups may occur in the region. Spawning patterns off eastern Australia were investigated using data collected during nine ichthyoplankton surveys conducted between 1997–2015, and adult reproductive data obtained from ad hoc commercial sampling off New South Wales (NSW). The egg surveys covered the known distribution of sardine off eastern Australia and included year‐round sampling in the northern and southern parts of this range. Egg distributions and analysis of gonadosomatic indices identified two spatio‐temporally separate spawning groups; one occurring off southern Queensland to northern NSW during late winter and early spring, and a smaller group off eastern Tasmania to southern NSW during summer. Most eggs were collected from waters 50–90 m deep, with sea surface temperatures of 18–23°C. Additive modelling indicated depth was the most significant factor driving selection of spawning habitat, followed by the interaction of month and latitude. Low egg densities were recorded in waters between 34–37°S, despite conditions within the ranges suitable for spawning. The presence of two spawning groups of sardine off eastern Australia supports recent findings that two sub‐populations occur in the region. Findings of this study will help to optimise the spatio‐temporal extent of future egg surveys and further confirm the need to coordinate future management of each sub‐population among relevant jurisdictions.

Distribution of Engraulis encrasicolus eggs and larvae in relation to coastal oceanographic conditions (the South-western Adriatic Sea case study)

Identification of potential spawning and nursery areas of European anchovy (Engraulis encrasicolus) represents an essential step in the management of a resource which is of fundamental importance both for fishery and pelagic trophic web. Egg and larvae occurrence from ichthyoplankton surveys (2012 - 2015) in the South Western Adriatic Sea were examined to understand the mechanisms that control their distribution. Egg and larvae densities varied through the years with the highest values recorded in 2012 and the lowest in 2014. Positive correlations between eggs and larvae with zooplankton were observed. When quotient analysis was used to find relations with environmental and biological variables, the results pointed out an egg preference for depth between 91 and 120 m and an avoidance between 11 and 30 m. Moreover, egg avoidances for high values of chlorophyll-a (> 0.52 mg m-3) and low values of zooplankton biomass (< 199 mg m-2) were highlighted. On the other hand, different ranges of habitat preferences and avoidances were observed for larvae: preference range of depth between 11 and 60 m and avoidance > 151 m; preference for high zooplankton biomass (> 1000 mg m-2) and avoidance for low biomass (< 299 mg m-2). These correlations and the quotient values suggest that egg and larvae distribution in the South-Western Adriatic Sea is mainly driven by food availability and depth.

Physical forcing on fish abundance in the southern California Current System

AbstractThe California Current System (CCS) is an eastern boundary current system with strong biological productivity largely due to seasonal wind‐driven upwelling and transport of the California Current (CC). Two independent, yet complementary time series, CalCOFI ichthyoplankton surveys and sampling of southern California power plant cooling‐water intakes, have indicated that an assemblage of predominantly cool‐water affinity fishes spanning nearshore to oceanic environments in the southern CCS has declined dramatically from the 1970s to the 2000s. We examined potential oceanographic drivers behind this decline both within and north of the CalCOFI survey area in order to capture upstream processes as well. Empirical orthogonal function (EOF) analyses using output from a data‐assimilative regional ocean model revealed significant relationships between the fish time series and spatial patterns of upwelling, upper ocean heat content and eddy kinetic energy in the CCS. Correlation and linear regression analyses indicated that the declining trend in fish abundance was correlated with a suite of factors: reduced offshore and increased inshore upwelling; a long term warming trend combined with more recent interannual variability in ocean temperature; weaker eddy kinetic energy north of Point Conception (35°N), potentially indicating reduced transport of the California Current (CC); increased influence of the California Undercurrent (CUC); and a decline in zooplankton displacement volume across the southern CCS. Understanding how changes in oceanography affect fish populations will offer insights into managing fisheries in a changing climate.

Life history and morphology of Eel Larvae in the Gulf of Guinea of western Africa: revisiting Jacques Blache’s research (1960–1977) 40 years later

Eel larvae (leptocephali) are rarely studied extensively both spatially and temporally, and detailed illustrations of most species are limited. This study uses the unique research reported in the monograph of Blache (Leptocephales des poissons anguilliformes dans la zone sud du golfe de Guinee. ORSTOM Faune Tropicale 10:1–381, 1977, in French) to describe and evaluate the species composition, abundance, life history characteristics and morphology of 10,284 anguilliform leptocephali collected throughout the year during 15 ichthyoplankton surveys (1960–1971) in relation to regional oceanography. Leptocephali of 70 species of 7 families were described, with Ophichthidae (26 species), Muraenidae (13), and Congridae (13) being the most diverse, and local spawning indicated by ≥ 34 species. Larvae of biogeographically restricted Heterencheylidae eels (mud eels) were abundant along the continental shelf and 5 species comprised 35% of total catches. Their larval distributions may reflect adult depth-segregation from nearshore/estuaries to the outer shelf and slope and larval retention. Nettastomatid leptocephali of Hoplunnis punctata were the most abundant species, and Rhynchoconger sp., Uroconger syringinus, Chlopsis olokun, and Dalophis boulengeri were also abundant. Small leptocephali distributions indicated spawning occurred over or near the continental shelf, and length-frequency data indicated most spawning was during the November–May warm-water season. Detailed morphology illustrations showed the characteristics of all stages of larvae. The Gulf of Guinea eel fauna is not diverse compared to the Indo-Pacific possibly due to phylogeography and a lack of coral reef habitats and the unusual low-latitude seasonal influx of cold surface waters, but is unique in being the worldwide center of distribution of the burrowing eels of the Heterencheylidae.

Distribution of the early life stages of small yellow croaker in the Yangtze River estuary and adjacent waters

The horizontal distribution of the early life stages of small yellow croaker Larimichthys polyactis in the Yangtze River estuary and its adjacent area was investigated through six ichthyoplankton surveys conducted during the period of mid-April to mid-July 2015. A total of seven eggs and 2461 larvae and juveniles of small yellow croaker were collected. Abundance of the specimens was highest in mid-May and lowest in mid-July. The composition of the different development stages of small yellow croaker varied among the six cruises, although post-flexion larvae and juveniles dominated the ichthyoplankton collection in each cruise. Specimens at different development stages showed different distribution patterns in the study area. The eggs, yolk-sac and pre-flexion larvae were mainly collected at stations with depth exceeding 30 m, while post-flexion larvae and juveniles were mainly distributed in the relatively shallow waters near to the mouth of the Yangtze River. In conclusion, the study suggested that the study area had largely lost the ecological function to support small yellow croaker spawning, although it could still serve as nursery ground for the species.

Characterizing larval swordfish habitat in the western tropical North Atlantic

AbstractSwordfish Xiphias gladius (Linnaeus, 1758) are a circumglobal pelagic fish targeted by multiple lucrative fisheries. Determining the distribution of swordfish larvae is important for indicating reproductive activity and understanding the early life history of swordfish. We identify and characterize larval swordfish distributions during peak swordfish spawning throughout the Gulf of Mexico and western Caribbean Sea with generalized additive models (GAMs) using catches of swordfish larvae during ichthyoplankton surveys in April and May of 2010, 2011, and 2012. The best fit GAM, as determined by stepwise, backward Akaike Information Criterion selection, included both physiochemical (temperature at 5 m, sea surface height anomaly (SSHA), eddy kinetic energy (EKE)), temporal (lunar illumination, hour of sampling) and spatial (location) variables, while near surface chlorophyll a concentration residuals remained as a random effect. The highest probability of larval swordfish catch occurred at sub‐surface temperatures, SSHA, and EKE values indicative of boundary currents. Standard lengths of larvae were larger further downstream in the boundary currents, despite high variability in length with location due to multiple spawning locations of swordfish near these currents. Probability of larval swordfish catch also peaked during the crescent and gibbous moons, indicating a lunar periodicity to swordfish spawning. These results suggest that swordfish may spawn during select moon phases near boundary currents that transport their larvae to larval and juvenile habitat including the northern Gulf of Mexico and coastal waters of the southeast United States.

Intra‐annual patterns of coastal larval fish assemblages along environmental gradients in the northeastern Mediterranean

AbstractTo date, little is known about the larval‐fish assemblages (LFAs) in the eastern Mediterranean. The purpose of this study is to investigate the intra‐annual variations of LFAs with an emphasis on the effects of physical, chemical, biological and meteorological conditions in the gulf of Iskenderun, which is a shallow (100 m maximum depth), semi‐enclosed bay located in the northeastern corner of the Mediterranean. The gulf is under the influence of offshore waters throughout the year. The study was based on an ichthyoplankton survey program performed at 28 stations over 12 months, between November 2009 and October 2010. During the study, a total of 11,411 larval individuals belonging to 177 taxa were sampled. The jackknife estimate of maximum species richness was 225 ± 19 for the study area. The maximum larval richness (84 species) and abundance (827 larvae/10 m2) values coincided with spring bloom in April. Gobiids dominated the ichthyoplankton of Iskenderun Bay by constituting 9% of species richness and 42% of total individuals. Four LFAs were identified which were in early winter (November to January), late winter (February and March), spring (April to June) and summer (July to October). The intra‐annual variations of richness, abundance and composition of ichthyoplankton were significantly correlated with the temperature and mesozooplankton biomass. The wind conditions and thermal stratification were also significantly correlated with the composition of LFAs. In conclusion, phenology of LFAs were formed under the influence of physical and tropho‐dynamic conditions in Iskenderun Bay, Northeastern Mediterranean.

Evaluating the relationship between spatial heterogeneity and temporal variability of larval fish assemblages in a coastal marine ecosystem (Haizhou Bay, China)

AbstractIn coastal marine ecosystems, spatial patterns of larval fish assemblages (LFAs) tend to exhibit geographic stability because of relatively stable spawning site selection and predictable oceanographic phenomena such as eddies. To evaluate the relationship between spatial heterogeneity and temporal variability of LFAs, we conducted a high spatiotemporal resolution ichthyoplankton survey from April to July in 2013 in the shallow waters (<20 m) of Haizhou Bay, China. Our analysis indicated three distinct assemblages, which were stable geographically but exhibited a gradual and directional change of species composition and abundance over our study period. Sea surface temperature was the most important environmental co‐variate for determining temporal variability of LFAs, likely owing to temperature effects of species composition and spawning period selection of adult fish, along with known temperature‐dependent survival rates of larval fish. Study of LFA spatiotemporal dynamics is essential for improved understanding of adult fish spawning behavior, and has potential to inform design and implementation of conservation and management measures (e.g. marine protected areas) in coastal systems.

Quantifying the influence of geography and environment on the northeast Atlantic mackerel spawning distribution

AbstractMackerel (Scomber scombrus) in the northeast Atlantic have shown changes in distribution at certain times of the year, which have impacted their exploitation and management. In this study, mackerel spawning habitat over 21 recent years was characterised using generalised additive modelling, based on spatial egg density data collected every third year during targeted ichthyoplankton surveys. Mackerel spawning distribution was found to depend primarily on geographical variables (coordinates and bottom depth), with preferred spawning locations on the shelf‐edge from the north of the Iberian peninsula to the west of Scotland, with a maximum west of Ireland. Environmental drivers had a lesser influence on egg distribution. Dome shaped relationships were found with temperature and mixed layer depth, with respective optimum at 13°C and around –300 m. The model was used to reconstruct maps of the potential habitat (areas where conditions were suitable, but not necessarily used, for spawning). Little changes were observed over the years in the potential habitat, suggesting that the expansion of the egg distribution (realised habitat) was not triggered by changes in the environmental variables investigated. Little evidence was found for density‐dependent habitat selection. There was a tendency for mackerel to make more use of areas of lesser suitability in years with large stock size (1992 and 2010). This pattern, however, broke down in 2013, when stock size was the highest, as spawning occurred very south and concentrated in the most suitable habitat.

Molecular identification of Bathymaster spp. (Bathymasteridae) larvae in the Gulf of Alaska and eastern Bering Sea

Information on the phenology and distribution of fish larvae is critical to understanding recruitment dynamics of populations. Larvae of the family Bathymasteridae (ronquils) are an abundant component of the ichthyoplankton community in the eastern North Pacific Ocean and Bering Sea, but cannot be readily identified to the species level using morphological or meristic approaches. We developed a restriction fragment length polymorphism (RFLP) protocol based upon variation in the mitochondrial (COI) gene to identify larvae of three species in the genus Bathymaster: smallmouth ronquil (B. leurolepis), Alaskan ronquil (B. caeruleofasciatus) and searcher (B. signatus). A subsample of larvae from annual ichthyoplankton surveys was screened using the RFLP protocol, and 259 of 260 individuals were correctly identified to species, providing the first data on the seasonal occurrence of early life-history stages in ichthyoplankton surveys for these species. Peak abundance for two species in surveys conducted from 2006 to 2014 occurred in spring. Larvae of smallmouth ronquil were absent from spring surveys and occurred as only 8% of the total numbers collected in later seasons. Our results indicate that larvae of Alaskan ronquil (B. caeruleofasciatus) were more likely to occur at deeper sampling stations (bottom depth > 180 m) in spring, while searcher (B. signatus) larvae were more common at shallower stations (bottom depth ≤ 100 m).

Ichthyoplankton community of the Dardanelles Strait, Turkey

Abstract An ichthyoplankton survey was conducted between March 2012 and February 2013 along the Anatolian coast of the Dardanelles Strait, with the aim of describing the species composition and temporal variation of fish eggs and larvae. For this purpose, monthly samples were collected at three sites using a WP-2 plankton net equipped with 500 µm mesh during a one-year period. Biodiversity indices were calculated to understand differences in biodiversity. A total of 50 teleost fish species belonging to 25 families and 7 orders were identified. The highest species richness of fish eggs was recorded in spring, whereas the highest species richness of fish larvae – in spring and summer. On the other hand, species richness of fish eggs and larvae were at a minimum in the fall. The most dominant species recorded in the Dardanelles were Sprattus sprattus as fish eggs and Engraulis encrasicolus as fish larvae. The relatively lower abundance of eggs and lack of earlier egg phases indicate that Engraulis encrasicolus does not spawn in the Dardanelles Strait, and the spawned eggs develop while drifting from the spawning grounds in the Marmara Sea to the sampling area.

Abundance and habitat associations of tuna larvae in the surface water of the Gulf of Mexico

Summer ichthyoplankton surveys were conducted in the northern Gulf of Mexico from 2007 to 2010 to characterize the distribution and abundance of tuna larvae. Larval assemblages of tunas were comprised of four genera: Thunnus, Auxis, Euthynnus, and Katsuwonus. Thunnus were the most abundant and four species were detected; T. atlanticus [blackfin tuna], T. obesus [bigeye tuna], T. albacares [yellowfin tuna], and T. thynnus [bluefin tuna]. Intra- and inter-annual variability in the distribution and abundance of Thunnus species were observed with higher densities in 2008 and 2009, with a decline in abundance observed in 2010. Distribution and abundance of Thunnus larvae were influenced by physical and chemical conditions of the water mass, notably sea surface temperature and salinity. Distinct species-specific habitat preferences were observed and the location of mesoscale oceanographic features influenced larval abundance with higher densities of T. atlanticus, T. obesus, and T. albacares near anticyclonic (warm core) regions and the Loop Current, while T. thynnus was observed in higher densities near cyclonic (cold core) regions. This study demonstrates that spatial and temporal variability in the location of mesoscale oceanographic features may be important to partitioning nursery habitat among Thunnus species.

Anchovy (Engraulis encrasicolus) early life stages in the Central Mediterranean Sea: connectivity issues emerging among adjacent sub-areas across the Strait of Sicily

The combined use of field data on anchovy (Engraulis encrasicolus, Linnaeus, 1758) egg distribution in the Central Mediterranean Sea on both sides of the Strait of Sicily (Sicilian–Maltese and Tunisian waters) and Lagrangian simulations were used to assess the pattern of connectivity between these two sub-areas as a result of spawning activity. The field data were collected during ichthyoplankton surveys carried out in summer 2008 and 2010. The simulation runs showed considerable (up to 20%) rates of particle exchange in both directions (from Tunisian to Sicilian–Maltese waters and vice versa). However, considering the typical high mortality rates of anchovy early stages, the actual larval exchange rates across the Sicily Strait are supposed to be significantly lower (<1%), supporting the hypothesis that the anchovy population sub-units in the Strait of Sicily can be considered as separate fish stocks for the evaluation of their optimum exploitation rates.

Using Fishermen’s Ecological Knowledge to map Atlantic cod spawning grounds on Georges Bank

AbstractThe spawning dynamics of Atlantic cod (Gadus morhua) on Georges Bank and Nantucket Shoals are not well understood. To address this uncertainty, we combined Fishermen’s Ecological Knowledge (FEK) with traditional scientific data to develop a more holistic understanding of cod spawning on Georges Bank. Data from historical reports, trawl surveys, fisheries observers, and ichthyoplankton surveys were used to describe the spatial and temporal distribution of cod spawning activity. We also collected FEK regarding cod spawning dynamics through semi-structured interviews (n = 40). The fishermen had detailed knowledge of the spatial and temporal distribution of cod spawning, and identified persistent fine-scale (i.e. &amp;lt;50 km2) spawning grounds that were often associated with specific habitat features, including spawning grounds that were previously unreported in the scientific literature. The spawning seasons and locations identified by fishermen generally agreed with information from traditional scientific data, but it was evident that seasonal scientific surveys lack the spatial and temporal resolution needed to fully characterize the distribution of cod spawning activity. Our results will help inform management measures designed to promote the rebuilding of Georges Bank cod, and also provide a basis for further investigations of cod spawning dynamics and stock structure.

Linking spawning ground extent to environmental factors — patterns and dispersal during the egg phase of four North Sea fishes

Previous studies have shown that four commercially important demersal species, namely Atlantic cod (Gadus morhua), haddock (Melanogrammus aeglefinus), whiting (Merlangius merlangus), and European plaice (Pleuronectes platessa), spawn in distinct areas across the North Sea. Based on two comprehensive ichthyoplankton surveys in 2004 and 2009, the present study uses generalized additive mixed models to delimit these spawning grounds using the distribution of recently spawned eggs, investigates their relationship to specific environmental conditions, and examines egg dispersal during their development. Results indicate that presence–absence of early stage eggs is more related to temporal and topographic variables, while egg densities are closely linked with hydrography. Egg distribution patterns were relatively consistent during development and only changed near hatching. Compared with historic observations, the location of the spawning grounds appeared stable on the broad scale but centres of egg abundance varied between the surveyed years. Potential effects of long-term climate change and anthropogenic short-term disturbances, such as seismic surveys, on fish reproduction are discussed, pointing out the demand for multispecies studies on these issues.

Incorporation of habitat information in the development of indices of larval bluefin tuna (Thunnus thynnus) in the Western Mediterranean Sea (2001–2005 and 2012–2013)

Fishery independent indices of bluefin tuna larvae in the Western Mediterranean Sea are presented utilizing ichthyoplankton survey data collected from 2001 through 2005 and 2012 through 2013. Indices were developed using larval catch rates collected using two different types of bongo sampling, by first standardizing catch rates by gear/fishing-style and then employing a delta-lognormal modeling approach. The delta-lognormal models were developed three ways: 1) a basic larval index including the following covariates: time of day, a systematic geographic area variable, month and year; 2) a standard environmental larval index including the following covariates: mean water temperature over the mixed layer depth, mean salinity over the mixed layer depth, geostrophic velocity, time of day, a systematic geographic area variable, month and year; and 3) a habitat-adjusted larval index including the following covariates: a potential habitat variable, time of day, a systematic geographic area variable, month and year. Results indicated that all three model-types had similar precision in index values. However, the habitat-adjusted larval index demonstrated a high correlation with estimates of spawning stock biomass from the previous stock assessment model, and, therefore, is recommended as a tuning index in future stock assessment models.