Fish morphology and passage through velocity barriers. Experience with northern straight-mouth nase (Pseudochondrostoma duriense Coelho, 1985) in an open channel flume

Knowing the relationship between size, morphological traits and swimming performance of fish is essential to understand the swimming capacity to successfully surpass these obstacles and the selective pressure that barriers in rivers and streams could exert on fish. Northern straight-mouth nase, an endemic potamodromous cyprinid fish species from the Northwest of the Iberian Peninsula, was selected to carry out volitionally swimming performance experiments in an open channel against three different flow velocities, using telemetry and video cameras. The use of thin-plate spline, on 10 landmarks, evidenced unknown patterns linked to velocity barriers. At lower flow velocity, size is the main factor explaining the swimming performance; thus, large individuals swim up more efficiently. In contrast, at high flow velocities, shape becomes the essential explanatory variable; thereby, streamlined body shapes with a higher relative position of the tail and a narrower caudal peduncle are more efficient. The obtained results show the existence of a relationship between fish morphology and swimming performance, with potential consequences due to selection pressures associated with velocity barriers and their implications on behavioural and dispersal processes. To sum up, velocity barriers could exert a selection pressure on nase populations, so the fishway design and removal should be (re)considered.

HOW TO COLLECT THE PICTURES ON THE FISH MORPHOLOGY.

Collecting fish morphology data in the field is usually difficult and time consuming. The method of digital photography with further computer processing is currently widely used in biology and facilitates the acquisition of morphological data of fish. The article gives recommendations for obtaining high-quality digital photographs of fish, which we have tested empirically on a large ichthyological material. The studies of the morphological variability of fish following these recommendations showed several positive aspects: accelerating the collection of morphological material, increase of the accuracy of the data obtained, organizing the material in the form of a data bank, the ability to repeatedly refer to a photograph of an object, and a quick exchange of material.

Do Morphological Traits Predict Ecological Guilds of the Mekong Fish Fauna?

Southeast Asian riverine fishes are classified into three guilds (‘black’, ‘white’ and ‘grey’ species) based on their reproductive and migration strategies. In this study, we aimed to investigate whether fish morphology could be used to predict the Mekong fish guilds. Nine dimensionless ratios of fish morphological traits were used to describe the locomotion and food acquisition strategies of 121 fish species. The links between morphological traits and fish guilds were assessed using a principal component analysis (PCA) and a variance partitioning analysis, which revealed a strong morphological overlap between the guilds. Despite the high contribution of intra-guild variability to overall morphological variability (~90%), black and white fish significantly differed in terms of locomotion-related traits. Mekong fish guilds were satisfactorily predicted by using a random forest (RF) model, which produced a percentage of successful classification of ca 50% for each of the three guilds. Caudal propulsion efficiency, pectoral fin vertical position and body elongation were the most significant traits in the RF predictive model. Although the present study provides initial insight into the links between Mekong fish morphology and ecological guilds, further research is needed in order to clarify the relationship between species morphology, migratory status and responses to environmental variation.

Prediction of square mesh panel and codend size selectivity of blue whiting based on fish morphology

Square mesh panels (SMPs) are selective devices used extensively to supplement codend size selectivity in trawl fisheries. Therefore, predictions of the effect of mesh size in both SMPs and codends on size selectivity are valuable. Here, we established a framework to predict the size selection of blue whiting through different SMPs and diamond mesh codends based on the morphological characteristics of this species. We hypothesized that size selection for an SMP is determined by different fish contact angles, whereas different mesh opening angles determine size selection for the codend. Based on these hypotheses, we first developed a model that enabled us to predict which sizes of blue whiting are able to pass through meshes of different sizes and shapes. We then tested whether the selectivity for blue whiting of the SMP, the codend, and the combination of both could be explained by the models. Finally, we predicted the size selectivity of multiple combinations of SMPs and diamond mesh codends. The method presented here can potentially be applied to make predictions for species other than blue whiting.

Diet and ecomorphology of predator fish species of the Amazonian floodplain lakes

Amazonian floodplain lakes host a high diversity of predatory fish which coexist and exploit the high diversity of available prey. Morphology could be the characteristic most closely associated with their preferred feeding sources (prey). However, it is unclear whether this association is direct or indirect. If it is indirect, swimming performance or preferential position in the water column could be the most evident characteristic. To examine the degree to which fish morphology of predator fish species is correlated to their dietary inputs, we compared the existence of morphological and feeding dissimilarity among eight predator species with the association between predator morphologies and preferred prey. We collected, measured, and sampled the stomach contents of fish from two lowland floodplain lakes associated with the Solimões River, Brazil, in May, August, and November of 2014. Of 187 collected fish across eight species, five species showed fish to be the most important item in their diets and three preferentially ate shrimp. Principal components analyses of ecomorphological attributes divided the species according to their ability to find the prey, swimming performance of the predator, and prey size. While there was significant distinction between the varying morphologies of predators, we were unable to distinguish between the specific diet of these species and did not find a correlation between morphology and feeding. These results are likely due to the fact that there is great abundance and diversity of available prey in the Amazonian floodplain lakes, so opportunistic feeding may be the primary foraging strategy of predator fish species living in these environments.

Target strength of skipjack tuna (Katsuwanus pelamis) associated with fish aggregating devices (FADs)

This paper presents measures of target strength (TS; dB re 1 m2) and models of TS vs. fork length (L; cm), i.e. TS=20logL+b20, for skipjack tuna associated with fish aggregating devices (FADs) in the Central Pacific Ocean. Measurements were made using 38-, 120-, and 200-kHz split-beam echosounders on a purse-seine workboat during fishing operations. To mitigate potential bias due to unresolved targets, TS measurements were rejected if they were not simultaneously detected with multiple echosounder frequencies in approximately the same location. The filtered TS and concomitantly sampled L data were used to estimate b20 = −76, −71, and −70.5 dB for 38, 120, and 200 kHz, respectively, using the method of least squares. For comparison, quasi-independent estimates of TS and b20 were calculated from acoustic echo-integration and catch data representing entire aggregations around the FADs. The results differed by ≤1 dB for all three frequencies. The sensitivities of these results to variations in fish morphology and behaviour were explored using a simulation of TS for fish without swimbladders. The utility of the results on acoustic properties of skipjack tuna and next research steps to achieve selective fishing at FADs are discussed.