Diel vertical distribution of postflexion larval Citharichthys spp. and Sebastes spp. off central California

The diel and spatial distributions of postflexion larval Citharichthys spp. and Sebastes spp. at 67 stations off central California were examined during March 1994 with a 10 m² MOCNESS during 24 h sampling operations. At each station, a CTD profiler with an attached PAR sensor was deployed to measure temperature, salinity, σ_t, dynamic height, and ambient light levels at depth. Catches of C. sordidus, C. stigmaeus, unidentified Sebastes spp., and S. jordani were larger at night than during the day and crepuscular periods, probably because of net avoidance during the day and crepuscular periods, and (for Citharichthys spp.) vertical migration into the upper water column at night. C. sordidus and C. stigmaeus migrated vertically through the pycnocline on a diurnal cycle, while Sebastes spp. apparently remained in the mixed layer above the pycnocline at all times. No definitive patterns were observed between horizontal contours of temperature, salinity, and σ_t at depth and catches of any larvae. This was attributed to the relatively homogeneous spatial pattern in the water column above the pycnocline, where the majority of the fish occurred.     

The feeding ecology of larval Pacific hake (Merluccius productus) in the California Current region: an updated approach using a combined OPC/MOCNESS to estimate prey biovolume

A combined optical particle counter (OPC) and multiple opening and closing net and environmental sensing system (MOCNESS) was used to obtain simultaneous measurements of the fine‐scale distribution of ‘prey‐sized’ particles and the vertical distribution of larvae of Pacific hake (Merluccius productus). Physical properties were also measured. The data were used to describe the feeding ecology of Pacific hake larvae, and to explore the effects of prey abundance, time of day, temperature and depth on feeding. Pacific hake larvae consumed a wide variety of prey including copepod eggs, nauplii, copepodites, and euphausiid metanauplii. Calanoid copepodites comprised > 75% of the ingested prey volume. First‐feeding larvae were 2.5–3.0 mm SL. These larvae consumed prey 40–200 μm wide. Larvae 3.0–6.5 mm long ingested prey 40–400 μm wide and larvae > 6.5 mm long ingested prey 400–700 μm wide. There were clear diel patterns in feeding incidence and prey volume ingested. Feeding commenced between 06:01 and 08:00 hours PST and continued until ～ 16:00 hours. Ingested prey items remained in the gut until complete gut evacuation occurred near dawn. The volume of food ingested was estimated using two indices, the cube root of the prey volume (CRPV) and the cube root of the stomach volume (CRSV). Standard length, log elapsed time since gut evacuation, depth and particle biovolume contributed significantly to variation in both indices. Temperature did not contribute to variability in either CRPV or CRSV.     

Effects of environmental variables on survey catch rates and distribution by size of shallow‐ and deep‐water Cape hakes,Merluccius capensis and Merluccius paradoxus off Namibia

In order to study the effects of temperature, oxygen, salinity and time of day on survey trawl catches, we modeled observed catches of juvenile, small, medium and large hakes per station as functions of zenith angle of the sun, geographical position, year, temperature, salinity, oxygen and depth. We used data from summer demersal surveys conducted during the period 2002–2015, together with a computation of the corresponding light level data from which the solar zenith angles were obtained, and fitted the generalized additive models to these data. Based on best model results, important covariates were oxygen, depth, geographical position and temperature. The best models explained 70%, 69%, 57% and 57% of the variability in catches of juvenile, small, medium and large Merluccius capensis, respectively, and 71%, 68%, 81% and 70% of juvenile, small, medium and large Merluccius paradoxus, respectively. The significant effects of temperature, oxygen, depth and geographical position on survey catches of hake of different size groups indicate that survey size structure may be affected by the behavior of both species towards environmental conditions. Greater care should therefore be taken when interpreting hake survey biomass estimates, based on swept area method, especially those that were collected during exceptional unfavourable environmental conditions. It would also be highly desirable if the oceanographic conditions are collected on each trawl station in order to improve understanding of the linkage between resources and environmental conditions.     

Interannual variation in pelagic juvenile rockfish (Sebastes spp.) abundance – going with the flow

We report results from 28 yr of a midwater trawl survey of pelagic juvenile rockfish (Sebastes spp.) conducted off the central California coast. The fishery‐independent survey is designed to provide pre‐recruit indices of abundance for use in groundfish stock assessments. Standardized catch rate time series for 10 species were developed from delta‐generalized linear models that include main effects for year, station, and calendar date. Results show that interannual fluctuations of all 10 species are strongly coherent but highly variable, demonstrating both high‐ and low‐frequency components. A similarly coherent result is observed in the size composition of fish, with large fish associated with elevated catch rates. In contrast, spatial and seasonal patterns of abundance show greater species‐specific differences. A comparison of the shared common trend in pelagic juvenile rockfish abundance, derived from principal components analysis, with recruitments from five rockfish stock assessments shows that the time series are significantly correlated. An examination of oceanographic factors associated with year‐to‐year variability indicates that a signature of upwelled water at the time of the survey is only weakly related to abundance. Likewise, basin‐scale indices (the Multivariate El Niño‐Southern Oscillation Index, the Pacific Decadal Oscillation, the North Pacific Gyre Oscillation, and the Northern Oscillation Index) are poorly correlated with abundance. In contrast, sea level anomalies in the months preceding the survey are well correlated with reproductive success. In particular, equatorward anomalies in the alongshore flow field following the spawning season are associated with elevated survival and poleward anomalies with poor survival.     

The vertical distribution of juvenile salmon (Oncorhynchus spp.) and associated fishes in the Columbia River plume

Simultaneous trawling at surface and at depth at one location off the Columbia River, Oregon, in June 2000 identified the depth distribution of juvenile salmonids and associated fishes. Juvenile salmon off the Columbia River were distributed primarily near the surface, within the upper 12 m. Highest densities of subyearling chinook salmon (Oncorhynchus tshawytscha) off the Columbia River were associated with high surface currents and decreasing tidal levels, with time of day possibly a co‐factor. Densities of yearling chinook salmon increased with higher turbidity. Pacific herring (Clupea pallasi) was the most abundant and commonly caught forage fish, with density increasing at night, probably related to diel vertical migration. Catches of juvenile salmonids were not associated with catches of forage fishes. Daytime surface trawling appears to be an appropriate method for assessing the distribution and abundance of juvenile salmonids in marine habitats.     

High abundance of larval rockfish over Cobb Seamount, an isolated seamount in the Northeast Pacific

The larval fish community in the region of Cobb Seamount (500 km west of Oregon) is dominated by myctophid species commonly encountered in the subarctic North Pacific. However, during a survey in June 1992, the ichthyoplankton community within 30 km of the seamount summit was almost completely dominated by larvae of various rockfish ( Sebastes ) species. Given their very small size (and hence very young age) and the fact that they occurred only rarely in samples collected > 30 km from the seamount summit, we conclude that these Sebastes larvae were produced locally over Cobb Seamount. Previous studies have shown that the Cobb fish fauna is dominated by various Sebastes spp. and that, unlike other fish present on the seamount, the rockfish populations may be self-recruiting. We suggest that a persistent clockwise (i.e. downwelling) eddy, consistent with a stratified Taylor cone, plays a critical role in retaining larval rockfish over Cobb Seamount and may contribute to the process of self-recruitment. The key to the success of rockfish on Cobb and other shallow Northeast Pacific seamounts seems to be linked to their viviparous life history.     

Fish assemblages in the Southern California Current: relationships with climate, 1951–2008

We examined the dominant patterns of variability in the fish fauna of the southern California Current based on a principal component (PC) analysis of the California Cooperative Oceanic Fisheries Investigations ichthyoplankton data set, 1951–2008. Eighty‐six taxa were analyzed, including all ecologically dominant fish species, both exploited and unexploited. The first three PCs accounted for 20.5, 12.4 and 6.8% of the variance of the data, respectively (total: 39.7%). Each was dominated by taxa from particular adult or larval habitats. PC 1 predominantly represented the coherent response of 24 mesopelagic taxa from 10 families and was most highly correlated with long‐term trends in midwater oxygen levels. PC 2 was dominated by six of the seven most abundant ichthyoplankton taxa in the region, predominantly California Current endemics including key pelagic species (northern anchovy, Pacific sardine and Pacific hake), rockfishes (genus Sebastes) and two midwater taxa. It was correlated primarily with sea surface temperature and exhibited a significant declining trend. PC 3 was dominated by coastal and reef‐associated fishes with predominantly southerly affinities. It was positively correlated with sea surface temperature and sea level height, a proxy for diminished flow of the California Current. The taxa dominating PCs 2 and 3 mostly spatially co‐occur as ichthyoplankton. These results suggest that fish assemblages in the California Current are predominantly influenced by environmental forcing of their ocean habitats as adults or larvae, or both.     

Effect of the lipid droplet adherence on growth and survival of the European hake (Merluccius merluccius) larvae

The effect of the adherence of the lipid droplet in the yolk‐sac on the growth and survival of the European hake (Merluccius merluccius) larvae was studied under starved and feeding conditions. Recently hatched hake larvae with adhered and non‐adhered lipid droplet (ALD and non‐ALD) were maintained during 8 days under starvation conditions. The adherence of the lipid droplet positively affected larval survival during starvation. At the end of the yolk consumption period (7–8 days after hatching), 90% of the surviving larvae had an ALD, while 10% of the survivors had a non‐ALD and showed a higher mortality rate. In a month feeding experiment, results show that the presence of non‐ALD in newly hatched hake larvae negatively affected their growth and survival. Dry weight and length (TL) of 20‐days‐old larvae with ALD were significantly higher than those with non‐ALD. Furthermore, lifespan of those larvae with non‐adhered lipid droplet was shorter, with most of them dying during the first 15–20 days of culture. As conclusion, the percentage of the lipid droplet adherence at hatching could be used as an indicator of ‘quality’ for European hake larvae: the greater the number of larvae with adhered lipid droplet, the better larval growth and survival.     

Recovery potential of black rockfish,Sebastes melanops Girard,recompressed following barotrauma

Overfished species of rockfish, Sebastes spp., from the Northeast Pacific experience high bycatch mortality because of ‘barotrauma’, a condition induced from the rapid change in pressure during capture. Field experiments show that it may be possible for rockfish to recover from barotrauma if quickly recompressed; however, no work has followed the physiological recovery of rockfish after recompression or determined whether it is possible for rockfish to survive barotrauma in the long term. Barotrauma was induced in adult black rockfish, Sebastes melanops Girard, from a simulated depth of 35 m, followed by recompression. Blood and selected tissues (eye, heart ventricle, head kidney, liver, rete mirabile and gonad) were sampled at days 3, 15 and 31 post‐recompression to evaluate the tissue‐ and physiologic‐level response during recovery. No mortality from barotrauma occurred during the experiments, and feeding resumed in 80% of both treatment and control fish. The primary injury in treatment fish was the presence of a ruptured swimbladder and/or a ruptured tunica externa (outer layer of swimbladder), which was slow to heal. Blood plasma was analysed for glucose, sodium, chloride, potassium, calcium, phosphorus, insulin‐like growth factor‐1 and cortisol. Plasma analyses indicated no strong effects because of barotrauma, suggesting overall handling stress outweighed any effect from barotrauma. Rockfish with ruptured swimbladders may face compromised competency in the wild; however, it appears the majority of black rockfish decompressed from 35 m have a high potential for recovery if recompressed immediately after capture. This research suggests recompression could be a valuable bycatch mortality reduction tool for rockfish in recreational fisheries.     

Chlorophyll a variation in the Kuroshio Extension revealed with a mixed-layer tracking float: implication on the long-term change of Pacific saury (Cololabis saira)

Variation of chlorophyll a from March 2004 to July 2005 in the formation region of Subtropical Mode Water in the Kuroshio Extension was observed with a mixed‐layer tracking profiling float parking at around 40 m depth. Chlorophyll a concentration in the mixed‐layer is seasonally high from winter to early spring (January–April) even in the deep mixed layer; whereas during winter it tends to be lower for the corresponding deeper mixed layer. The chlorophyll a integrated over the winter mixed‐layer depth (MLD) increases with MLD for MLD <200 m and for MLD > 250 m the integrated chlorophyll a almost disappears, probably because of the large MLD exceeds in the critical depth with light limitation. These results suggest that the wintertime MLD influences the recruitment of Pacific saury (Cololabis saira) in the Kuroshio Extension because the wintertime food environment for larvae and juvenile of the Pacific saury could be better in the shallow wintertime MLD period of the 1950s and 1990s and worse in the deep MLD period of the 1970s and early 1980s.     

Migration and behavior of juvenile North Pacific albacore (Thunnus alalunga)

Archival tags were used to study the seasonal movements, migration patterns and vertical distribution of juvenile North Pacific albacore (Thunnus alalunga). Between 2001 and 2006, archival tags were deployed in North Pacific albacore in two regions of the Northeast Pacific: (i) off Northern Baja California, Mexico and Southern California, and (ii) off Washington and Oregon. Twenty archival tagged fish were recovered with times at liberty ranging from 63 to 697 days. Tagged albacore exhibited five distinct, seasonal migratory patterns. Depth and temperature data also showed a broad range of vertical behaviors. In certain regions such as off Baja California, Mexico, juvenile albacore make frequent dives to depths exceeding 200 m during the day and remain in the surface mixed layer at night, whereas off Oregon and Washington they remain near the surface both day and night. Water temperatures encountered ranged from 3.3 to 22.7°C. Peritoneal temperatures were significantly higher by an average of approximately 4°C, as expected in these warm‐bodied fish. This study provides a comprehensive examination of horizontal and vertical movements of juvenile albacore in the Northeast Pacific. The results reveal diverse behavior that varies regionally and seasonally as albacore move among different habitats throughout the entire North Pacific.     

Biomass and taxonomic composition of micronekton in the western tropical–subtropical Pacific

The biomass distribution, community structure and species composition of the dominant micronektonic animals were determined in the western tropical Pacific, using day/night sampling from upper 200 m depth with a commercial midwater otter trawl in fall‐winter of 1994, 1995 and 1997. Night‐time micronekton biomass was significantly higher than daytime biomass except for a few cases. All the night‐time catches had similar taxonomic composition, dominated by myctophid fishes, small squids, shrimps and euphausiids. Night‐time micronekton biomass in the upper 200 m was assumed to be higher in the North Equatorial Counter Current than in the North Equatorial Current, reflecting the zooplankton biomass distribution which micronektons feed on at night. A total of 42 species of myctophid fishes, 34 species of squids, 27 species of shrimps and six species of euphausiids were collected. Community types of each dominant taxonomic group were classified by cluster analysis based on the species composition. A station located north of the subtropical convergence zone was distinct from other tropical stations south of the convergence zone, having significantly different species composition in three of the four dominant taxonomic groups (myctophids, squids and shrimps). The interaction between the larvae of commercially important fishes and micronektonic animals is discussed.     

Establishing climate–growth relationships for yelloweye rockfish (Sebastes ruberrimus) in the northeast Pacific using a dendrochronological approach

We applied dendrochronology (tree‐ring) methods to develop multidecadal growth chronologies from the increment widths of yelloweye rockfish (Sebastes ruberrimus) otoliths. Chronologies were developed for the central California coast, a site just north of Vancouver Island, British Columbia, and at Bowie Seamount west of the Queen Charlotte Islands, British Columbia. At each site, synchronous growth patterns were matched among otoliths via the process of cross‐dating, ensuring that the correct calendar year was assigned to all increments. Each time series of growth‐increment measurements was divided by the values predicted by a best‐fit negative exponential function, thereby removing age‐related trends. These detrended time series were averaged into a master chronology for each site, and chronologies were correlated with monthly averages of sea surface temperatures, upwelling, the Northern Oscillation Index, and the Pacific Decadal Oscillation. The two northern growth chronologies positively correlated with indices of warm ocean conditions, especially from the prior summer through the spring of the current year. During the same period, the California chronology positively correlated with indices of cool ocean conditions, indicating an opposing productivity regime for yelloweye rockfish between the California Current and the Gulf of Alaska. Overall, this study demonstrates how tree‐ring techniques can be applied to quickly develop annually resolved chronologies and establish climate–growth relationships across various temporal and spatial scales.     

Atmospheric forcing drives recruitment variation in the Dungeness crab (Cancer magister), revisited

For 12 yr (1997–2001, 2006–2012) daily abundance of Cancer magister megalopae was measured in Coos Bay, Oregon. Before 2007 from 2000 to 80 000 megalopae were caught annually. In 2007, catch jumped and has since varied from 164 000 to 2.3 million. The step change in catch size appears related to a shift to negative Pacific Decadal Oscillation (PDO) values. Late season catches, which cannot be due to local spawning, are negatively correlated to the PDO, suggesting that these megalopae derive from north of the California Current. During periods of lower and higher catches, annual returns of megalopae were significantly negatively correlated to the day of the year of the spring transition and positively correlated to the amount of upwelling during the settlement season. The size of the Oregon commercial catch lagged 4 yr to allow for growth of recruits into the fishery is set by the number of returning megalopae; the relationship is parabolic. At lower returns, the population is recruitment limited, but at higher returns, density‐dependent effects predominate and set the commercial catch. Lagged commercial catches in Washington and Northern and Central California were also related to the number of megalopae returning to Coos Bay, suggesting that the forces causing variation in larval success are coast wide. If high return rates are due to a PDO regime shift, then for years to decades the commercial catch may be set by density‐dependent effects following settlement and the huge numbers of returning megalopae may impact benthic community structure.     

Bycatch and discarding in the South African demersal trawl fishery

Observers aboard commercial trawlers collected data on the total catch composition of 614 and 479 hauls made by vessels operating off the south and west coasts of South Africa, respectively. On the south coast, four fishing areas were identified on the basis of target species and fishing depth. On the west coast, hauls were separated into those targeting hake Merluccius spp. in four depth ranges (0–300, 301–400, 401–500, and >500 m) and those targeting monkfish Lophius vomerinus. For each area, the catch composition was calculated and the species assemblages were investigated using cluster analysis and multi-dimensional scaling. Finally, for each coast, the weight of fish discarded annually was estimated. On the south coast, although hake dominated, between 21% and 47% of the catch was not hake, depending on the fishing area. In comparison, hake dominated west coast catches, the proportion of hake increasing with depth. For each fishery investigated, approximately 90% of the catch was processed and landed. However, estimates of annual discards indicate that the south and west coast fisheries may annually discard 9000 or 10,000 t and 17,000 or 25,000 t, of undersized and unutilizable fish and offal, respectively, depending on the estimation method used. When developing strategies to limit or enhance utilization of bycatch, cognisance should be taken of the differences in catch composition between the south and west coasts and of the importance of bycatch revenue to south coast fishing companies.     

Movements and behaviors of swordfish in the Atlantic and Pacific Oceans examined using pop‐up satellite archival tags

Swordfish are highly specialized top‐level predators that have been challenging to study. In this paper, data from 31 pop‐up satellite archival tags attached to swordfish from (i) the eastern Pacific, (ii) central Pacific, and (iii) western North Atlantic‐Caribbean were analyzed. Common across locations was a pronounced diel vertical pattern with daytime hours spent primarily below the thermocline and nighttime hours spent in warmer waters, close to the surface. One exception to this pattern was periodic daytime basking events which were most common in cooler waters off California. Maximum daytime depths were significantly correlated with light penetration as measured by the diffuse attenuation coefficient at 490 nm. Temperature did not appear to influence daytime depths, and swordfish tolerated both extremely low temperatures (4°C) and rapid and dramatic temperature changes (>20°C). Temperature did appear to influence the nighttime depths in the Pacific where fish typically remained in the surface mixed layer. In contrast, in the warm tropical Atlantic this was not the case, and nighttime depths were much deeper. In all areas, nighttime depth increased around the full moon. Given the parallels between the vertical movement patterns of swordfish and those of the deep sound scattering layer we suggest that swordfish vertical distribution patterns, especially during daytime, are influenced largely by resource availability. At night, when swordfish are typically targeted by fisheries, both ambient light and temperature influence movements. Understanding vertical movement patterns of swordfish can help evaluate gear vulnerability, improve population assessments, and potentially reduce fisheries bycatch.     

Occurrence of demersal fishes in relation to near‐bottom oxygen levels within the California Current large marine ecosystem

Various ocean‐climate models driven by increased greenhouse gases and higher temperatures predict a decline in oceanic dissolved oxygen (DO) as a result of greater stratification, reduced ventilation below the thermocline, and decreased solubility at higher temperatures. Since spreading of low oxygen waters is underway and predicted to increase, understanding impacts on higher trophic levels is essential. Within the California Current System, shoaling of the oxygen minimum zone (OMZ) is expected to produce complex changes. Onshore movement of the OMZ could lead to habitat compression for species with higher oxygen requirements while allowing expansion of species tolerant of low bottom DO. As part of annual groundfish surveys, we sampled catch across a range of conditions from the upper to the lower limit of the OMZ and shoreward across the continental shelf of the US west coast. DO ranged from 0.02 to 4.25 mL L^?1 with 642 stations (of 1020 sampled) experiencing hypoxic conditions in 2008–2010. Catch and species richness exhibited significant and positive relationships with near‐bottom oxygen concentration. The probability of occurrence was estimated for four species (spotted ratfish, petrale sole, greenstriped rockfish and Dover sole) using a binomial Generalized Additive Model. The models for each species included terms for position, day of the year, salinity, near‐bottom temperature and the interaction term between depth and near‐bottom DO. Spotted ratfish and petrale sole were sensitive to changes in near‐bottom oxygen, while greenstriped rockfish and Dover sole show no changes in probability of occurrence in relation to changes in oxygen concentration.     

Diets of top predators indicate pelagic juvenile rockfish (Sebastes spp.) abundance in the California Current System

Diets of top predators may be useful indicators to the availability of forage fish in marine ecosystems. Juvenile rockfish (young‐of‐the‐year Sebastes spp.) compose a significant part of the diet for many predators in the central California Current, including chinook salmon (Oncorhynchus tshawytscha), and several species of marine birds and mammals. Herein, we develop annual indices of juvenile rockfish relative abundance by collating time series data sets on: (i) the proportion of rockfish in the diet of three species of seabirds breeding on Southeast Farallon Island (1975–2002); (ii) the number of rockfish in chinook salmon stomachs (1980–99); and (iii) the abundance of rockfish captured in scientific mid‐water trawl net surveys (1983–2002). We used Principal Component Analysis (PCA) to combine indices, and refer to these as ‘Multivariate Rockfish Indices’ (MRI). Combining time series verifies the patterns shown by each alone and provides a synoptic perspective on juvenile rockfish relative abundance. The diets of predators with the largest foraging ranges (Common Murre, Uria aalge) and chinook salmon co‐varied strongly with the net samples, and appear to be the best indicators. The salmon also sampled species of Sebastes not caught in the nets. The MRI reveals interannual variability in juvenile rockfish abundance, a substantial decline in abundance in the 1990s, and a partial recovery in the early 2000s. Predator‐based sampling is a cost‐effective enhancement of scientific net sampling.     

The population dynamics of the longnose skate,Raja rhina,in the northeast Pacific Ocean

The longnose skate, Raja rhina, is the most commonly landed skate species in the northeast Pacific Ocean. It also dominates survey catches in the area. Existing knowledge on skate biology and fisheries suggests that large skate species with late sexual maturation, like the longnose skate, are extremely vulnerable to overfishing, and if overfished they are slow to recover. Historically, in the US Pacific Coast skates have not supported directed fisheries, but have been taken as bycatch in other commercially important fisheries. For the past 13 years, landed catches of skates in waters off Washington, Oregon and California have significantly increased. Using the Stock Synthesis 2 modeling framework, a population model for the longnose skate was developed to assess the current state of the stock and reconstruct its past dynamics. Model results indicate that the longnose skate spawning biomass has slowly declined since 1915; the current stock biomass is estimated as 66% of its unexploited level. Uncertainty in regards to fishery-dependent data and female maturity parameters was explored through sensitivity analyses. The model results indicate that it is important to conduct species-specific identification in the fishery and monitor discard of longnose skate to improve the accuracy of fishery catch data and make assessment conclusions more robust.     

Effectiveness of shore‐based remote camera monitoring for quantifying recreational fisher compliance in marine conservation areas

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