Joseph J. Bizzarro

Comparative feeding ecology of four sympatric skate species off central California, USA

The big (Raja binoculata), California (R. inornata), longnose (R. rhina), and sandpaper (Bathyraja kincaidii), skates are commonly found on soft-bottom regions of the central California continental shelf and upper slope. The feeding ecology of this assemblage was compared to evaluate the degree of trophic separation among species, based on the results of previous species-specific diet studies. Specimens were collected from fishery independent trawl surveys conducted during September 2002–March 2003 at depths of 9–536 m. Using single and compound measures, diet composition of small (≤60 cm TL) and large (>60 cm TL) individuals were compared within continental shelf (≤200 m) and slope (>200 m) regions using traditional, multivariate, and novel techniques. Diet compositions within size classes were similar in both regions. Diet compositions between size classes generally differed, however, with fishes more important and crustaceans (especially shrimps and euphausiids) less important in the diets of large individuals. Crabs contributed substantially to skate diet compositions on the shelf, but were uncommon prey items at deeper depths, probably because of their relative scarcity in slope waters. Conversely, cephalopods were common prey items at slope depths, but were rarely ingested at shelf depths. The studied skate assemblage appears to consist primarily of generalist crustacean and fish predators that exhibit high dietary overlap at similar sizes. It is possible that resource competition among skates and groundfish species has been reduced because of considerable recent declines in the biomass of upper trophic level groundfishes. Skates may therefore play important roles in contemporary benthic food web dynamics off central California.

The evolution of cranial design, diet, and feeding mechanisms in batoid fishes

The batoid fishes (electric rays, sawfishes, skates, guitarfishes, and stingrays) are a trophically and morphologically diverse clade in which the observed range of diets is a product of a feeding mechanism with few parts and therefore a limited number of functional interactions. This system allows an intriguing comparison to the complex network of associations in the feeding apparatus of bony fishes and an anatomically simple framework for investigations of the mechanisms underlying the evolution of functional and phenotypic diversity. We quantified morphology from reconstructed CT scans of 40 batoid species, representing more than half of the extant genera. We used pairwise comparisons to evaluate the extent of coevolution among components of the feeding apparatus and among morphologies and diets. These relationships were then used to predict diets in poorly studied taxa and in a reconstruction of the batoid ancestor. Although functionally there are fewer examples of convergence in the batoid feeding mechanism than in bony fishes, our data show multiple evolutions of similar dietary compositions underlain by a broad morphological diversity. Elements of the feeding apparatus evolved independently of one another, suggesting that decoupling components of the head skeleton created separate but interacting evolutionary modules that allowed trophic diversification. Our data imply that food habits exhibit strong independent and convergent evolution and that suites of morphologies are associated with certain diets; however, lack of behavioral data for this clade, and one example of divergent diets underlain by convergent morphology, caution against the assumption of simplistic relationships between form and function. We therefore urge future work to ground truth our study by testing the functional, dietary and evolutionary hypotheses suggested by our data.

Breaking with tradition: redefining measures for diet description with a case study of the Aleutian skate Bathyraja aleutica (Gilbert 1896)

Characterization of fish diets from stomach content analysis commonly involves the calculation of multiple relative measures of prey quantity (%N,%W,%FO), and their combination in the standardized Index of Relative Importance (%IRI). Examining the underlying structure of dietary data matrices reveals interdependencies among diet measures, and obviates the advantageous use of underused prey-specific measures to diet characterization. With these interdependencies clearly realized as formal mathematical expressions, we proceed to isolate algebraically, the inherent bias in %IRI, and provide a correction for it by substituting traditional measures with prey-specific measures. The resultant new index, the Prey-Specific Index of Relative Importance (%PSIRI), is introduced and recommended to replace %IRI for its demonstrated more balanced treatment of the relative measures of prey quantity, and less erroneous behavior across taxonomic levels of identified prey. As a case study, %PSIRI was used to examine the diet of the Aleutian skate Bathyraja aleutica from specimens collected from three ecoregions of the northern Gulf of Alaska (GOA) continental shelf during June-September 2005–2007. Aleutian skate were found to primarily consume the commonly abundant benthic crustaceans, northern pink shrimp Pandalus eous and Tanner crab Chionoecetes bairdi, and secondarily consume various teleost fishes. Multivariate variance partitioning by Redundancy Analysis revealed spatially driven differences in the diet to be as influential as skate size, sex, and depth of capture. Euphausiids and other mid-water prey in the diet were strongly associated with the Shelikof Strait region during 2007 that may be explained by atypical marine climate conditions during that year.

Bottles as models: predicting the effects of varying swimming speed and morphology on size selectivity and filtering efficiency in fishes

SUMMARY We created physical models based on the morphology of ram suspension-feeding fishes to better understand the roles morphology and swimming speed play in particle retention, size selectivity and filtration efficiency during feeding events. We varied the buccal length, flow speed and architecture of the gills slits, including the number, size, orientation and pore size/permeability, in our models. Models were placed in a recirculating flow tank with slightly negatively buoyant plankton-like particles (∼20–2000 μm) collected at the simulated esophagus and gill rakers to locate the highest density of particle accumulation. Particles were captured through sieve filtration, direct interception and inertial impaction. Changing the number of gill slits resulted in a change in the filtration mechanism of particles from a bimodal filter, with very small (≤50 μm) and very large (>1000 μm) particles collected, to a filter that captured medium-sized particles (101–1000 μm). The number of particles collected on the gill rakers increased with flow speed and skewed the size distribution towards smaller particles (51–500 μm). Small pore sizes (105 and 200 μm mesh size) had the highest filtration efficiencies, presumably because sieve filtration played a significant role. We used our model to make predictions about the filtering capacity and efficiency of neonatal whale sharks. These results suggest that the filtration mechanics of suspension feeding are closely linked to an animals swimming speed and the structural design of the buccal cavity and gill slits.

A Review of the Physical and Biological Characteristics of the Bahia Magdalena Lagoon Complex (Baja California Sur, Mexico

Abstract The Bahía Magdalena lagoon complex (BMLC) is an extremely productive and biologically diverse embayment on the Pacific coast of Baja California Sur, Mexico, and one of the most important fishing ports in the state. Local hydrologic conditions are largely determined by the differential influence of the California Current and California Countercurrent, with upwelling also affecting the physical characteristics of west–central Bahía Magdalena. Because of its subtropical location and variable hydrology, the BMLC is considered a transitional zone between temperate and tropical faunal regions. Tropical species are dominant among most taxa, especially in Bahía Almejas, but species composition is highly variable and coupled with environmental conditions.

Activities and Catch Composition of Artisanal Elasmobranch Fishing Sites on the Eastern Coast of Baja California Sur, Mexico

Abstract Eighty–three artisanal fishing sites were documented from seasonal surveys of the Gulf of California coast of Baja California Sur conducted during El Niño (1998) and La Niña (1999) conditions. The direct targeting of elasmobranchs was observed at approximately half (48.2%) of these sites. Sharks numerically dominated sampled landings (71.3%, n  =  693), and exceeded those of batoids during all seasons. Among the primary species in observed landings were the scalloped hammerhead, Sphyrna lewini (15.2%, n  = 148), Pacific angel shark, Squatina californica (11.6%, n  =  113), blue shark, Prionace glauca (11.4%, n  =  111), Pacific sharpnose shark, Rhizoprionodon longurio (11.3%, n  =  110), and pygmy devil ray, Mobula munkiana (8.6%, n  =  84).

Spatial segregation in eastern North Pacific skate assemblages.

Skates (Rajiformes: Rajoidei) are common mesopredators in marine benthic communities. The spatial associations of individual species and the structure of assemblages are of considerable importance for effective monitoring and management of exploited skate populations. This study investigated the spatial associations of eastern North Pacific (ENP) skates in continental shelf and upper continental slope waters of two regions: central California and the western Gulf of Alaska. Long-term survey data were analyzed using GIS/spatial analysis techniques and regression models to determine distribution (by depth, temperature, and latitude/longitude) and relative abundance of the dominant species in each region. Submersible video data were incorporated for California to facilitate habitat association analysis. We addressed three main questions: 1) Are there regions of differential importance to skates?, 2) Are ENP skate assemblages spatially segregated?, and 3) When skates co-occur, do they differ in size? Skate populations were highly clustered in both regions, on scales of 10s of kilometers; however, high-density regions (i.e., hot spots) were segregated among species. Skate densities and frequencies of occurrence were substantially lower in Alaska as compared to California. Although skates are generally found on soft sediment habitats, Raja rhina exhibited the strongest association with mixed substrates, and R. stellulata catches were greatest on rocky reefs. Size segregation was evident in regions where species overlapped substantially in geographic and depth distribution (e.g., R. rhina and Bathyraja kincaidii off California; B. aleutica and B. interrupta in the Gulf of Alaska). Spatial niche differentiation in skates appears to be more pronounced than previously reported.

Diet Composition and Trophic Ecology of Northeast Pacific Ocean Sharks

Although there is a general perception of sharks as large pelagic, apex predators, most sharks are smaller, meso- and upper-trophic level predators that are associated with the seafloor. Among 73 shark species documented in the eastern North Pacific (ENP), less than half reach maximum lengths >200cm, and 78% occur in demersal or benthic regions of the continental shelf or slope. Most small (≤200cm) species (e.g., houndsharks) and demersal, nearshore juveniles of larger species (e.g., requiem sharks) consume small teleosts and decapod crustaceans, whereas large species in pelagic coastal and oceanic environments feed on large teleosts and squids. Several large, pelagic apex predator species occur in the ENP, but the largest species (i.e., Basking Shark, Whale Shark) consume zooplankton or small nekton. Size-based dietary variability is substantial for many species, and segregation of juvenile and adult foraging habitats also is common (e.g., Horn Shark, Shortfin Mako). Temporal dietary differences are most pronounced for temperate, nearshore species with wide size ranges, and least pronounced for smaller species in extreme latitudes and deep-water regions. Sympatric sharks often occupy various trophic positions, with resource overlap differing by space and time and some sharks serving as prey to other species. Most coastal species remain in the same general region over time and feed opportunistically on variable prey inputs (e.g., season migrations, spawning, or recruitment events), whereas pelagic, oceanic species actively seek hot spots of prey abundance that are spatiotemporally variable. The influence of sharks on ecosystem structure and regulation has been downplayed compared to that of large teleosts species with higher per capita consumption rates (e.g., tunas, billfishes). However, sharks also exert indirect influences on prey populations by causing behavioural changes that may result in restricted ranges and reduced fitness. Except for food web modelling efforts in Alaskan waters, the trophic impacts of sharks are poorly incorporated into current ecosystem approaches to fisheries management in the NEP.

Large batoid fishes frequently consume stingrays despite skeletal damage

The shapes of vertebrate teeth are often used as hallmarks of diet. Here, however, we demonstrate evidence of frequent piscivory by cartilaginous fishes with pebble-like teeth that are typically associated with durophagy, the eating of hard-shelled prey. High-resolution micro-computed tomography observation of a jaw specimen from one batoid species and visual investigation of those of two additional species reveal large numbers of embedded stingray spines, arguing that stingray predation of a scale rivalling that of the largest carnivorous sharks may not be uncommon for large, predatory batoids with rounded, non-cutting dentition. Our observations demonstrate that tooth morphology is not always a reliable indicator of diet and that stingray spines are not as potent a deterrent to predation as normally believed. In addition, we show that several spines in close contact with the jaw skeleton of a wedgefish (Rhynchobatus) have become encased in a disorganized mineralized tissue with a distinctive ultrastructure, the first natural and unequivocal evidence of a callus-building response in the tessellated cartilage unique to elasmobranch skeletons. Our findings reveal sampling and analysis biases in vertebrate ecology, especially with regard to the role of large, predatory species, while also illustrating that large body size may provide an escape from anatomical constraints on diet (e.g. gape size, specialist dentition). Our observations inform our concepts of skeletal biology and evolution in showing that tessellated cartilage—an ancient alternative to bone—is incapable of foreign tissue resorption or of restoring damaged skeletal tissue to its original state, and attest to the value of museum and skeletal specimens as records of important aspects of animal life history.