Hans Polet

Reducing the benthos by-catch in flatfish beam trawling by means of technical modifications

In the flatfish beam trawl fisheries the by-catch by weight of invertebrates is several times the amount of marketable fish. In order to reduce the impact of beam trawling on the benthic communities, a number of benthos escape modifications to the trawl were tested. A drop-out opening (escape zone without netting) and large diamond and square mesh escape zones just behind the groundrope were not effective in releasing the benthos by-catch and induced an unacceptable decrease in commercial catch. Square mesh windows inserted in the belly just in front of the cod-end were more promising. With these devices a significant reduction in weight and number of most benthic species could be realised. The penalty is some loss of commercial catch but the results indicate that with an appropriate mesh size in the square mesh window, a balance may be found between a significant benthos by-catch reduction and an acceptable loss of marketable fish.

Vibrio tapetis isolated from vesicular skin lesions in Dover sole (Solea solea)

Vibrio tapetis is primarily known as the causative agent for brown ring disease in bivalves, although it has been isolated from cultivated fish during mortalities on farms. Here we describe the first isolation of V. tapetis from wild-caught and subsequently captive-held Dover sole Solea solea. Pathological features consisted of multifocal circular greyish-white skin discolourations evolving into vesicular lesions and subsequent ulcerations on the pigmented side. On the non-pigmented side, multiple circular lesions-white at the center and red at the edges-were evident. Histological examination of the vesicular lesions revealed dermal fluid-filled spaces, collagen tissue necrosis and a mixed inflammatory infiltrate, with large numbers of small rod-shaped bacteria. In the deep skin lesions, loss of scales and dermal connective tissue, with degeneration and fragmentation of the myofibres bordering the ulceration, were noted. Serotyping, DNA-DNA hybridization and REP- and ERIC-PCR techniques showed that the retrieved isolates displayed a profile similar to the representative strain of genotype/serotype O2 which originally was isolated from carpet-shell clam Venerupis decussata and to which isolates obtained from wedge sole Dicologoglossa cuneata were also closely related.

Atlantic Cod Show a Highly Variable Sensitivity to Electric-Induced Spinal Injuries

Abstract Pulse trawling is the most promising alternative to conventional beam trawls targeting Sole Solea solea (also known as Solea vulgaris), but due to the electric fields created by electrotrawls spinal injuries are reported in gadoid round fishes such as Atlantic Cod Gadus morhua. This study aimed to investigate the variability in the occurrence of electric-induced spinal injuries in cod. Four groups of cod, each originating from a different wild or farmed stock, were exposed to the pulses used by electrotrawls targeting Sole. Effects were analyzed based on behavior, mortality, and lesions up to 14 d after exposure, and morphological characteristics such as size, somatic weight, muscularity, the number of vertebral bodies, and the vertebral mineral contents of animals were compared among different cohorts. Second, the influence of factors such as water temperature, electrode diameter, and pulse type and amplitude were tested. Electrode diameter and pulse amplitude showed a positive correlation with the intensity of the fishs reaction. However, the present experiments confirmed that cod also show variable vulnerability, with injury rates ranging from 0% to 70% after (almost) identical exposures near the electrode. This indicates that these injuries are not only determined by the pulse parameter settings but also by subtle, fish-specific factors. Although the absence of a sensitive group of cod did not enable the elucidation of the conclusive factor, the effect of physiological and morphological factors such as intervertebral ligaments and rearing conditions during early life merit further attention in future research.

Laboratory Study of the Impact of Repetitive Electrical and Mechanical Stimulation on Brown Shrimp Crangon crangon

Abstract Pulse trawling is currently the best available alternative to beam trawling in the brown shrimp Crangon crangon and Sole Solea solea (also known as Solea vulgaris) fisheries. To evaluate the effect of repetitive exposure to electrical fields, brown shrimp were exposed to the commercial electrodes and pulse settings used to catch brown shrimp (shrimp startle pulse) or Sole (Sole cramp pulse) 20 times in 4 d and monitored for up to 14 d after the first exposure. Survival, egg loss, molting, and the degree of intranuclear bacilliform virus (IBV) infection were evaluated and compared with those in stressed but not electrically exposed (procedural control) and nonstressed, nonexposed (control) brown shrimp as well as brown shrimp exposed to mechanical stimuli. The lowest survival at 14 d (57.3%) occurred in the Sole cramp pulse treatment, and this was significantly lower than in the group with the highest survival, the procedural control (70.3%).No effect of electrical stimulation on the severity of IBV infection was found. The lowest percentage of molts occurred in the repetitive mechanical stimulation treatment (14.0%), and this was significantly lower than in the group with the highest percentage of molts, the procedural control (21.7%). Additionally, the mechanically stimulated brown shrimp that died during the experiment had a significantly larger size than the surviving individuals. Finally, no effect of the shrimp startle pulse was found. Therefore, it can be concluded that repetitive exposure to a cramp stimulus and mechanical stimulation may have negative effects on the growth and/or survival of brown shrimp. However, there is no evidence that electrical stimulation during electrotrawls would have a larger negative impact on brown shrimp stocks than mechanical stimulation during conventional beam trawling.

Impact of Pulsed Direct Current on Embryos, Larvae, and Young Juveniles of Atlantic Cod and its Implications for Electrotrawling of Brown Shrimp

AbstractThe application of electrical pulses in fishing gear is considered a promising option to increase the sustainability of demersal trawl fisheries. In the electrotrawl fishery for brown shrimp Crangon crangon, an electrical field selectively induces a startle response in the shrimp. Other benthic organisms remain mainly on the seafloor and escape underneath a hovering trawl. Previous experiments have indicated that this pulse has no short-term major harmful effects on adult fish and invertebrates. However, the impact on young marine life stages is still unknown. Because brown shrimp are caught in shallow coastal zones and estuaries, which serve as important nurseries or spawning areas for a wide range of marine species, electrotrawling on these grounds could harm embryos, larvae, and juveniles. We carried out experiments with different developmental stages of Atlantic Cod Gadus morhua, which are considered vulnerable to electrical pulses. Three embryonic stages, four larval stages, and one juvenile ...

Climate change: Threat or opportunity for Belgian sea fisheries?

Sea fisheries are an important activity for the coastal community of Flanders from a socio-cultural-economic point of view. It’s apparent that the Belgian fishing fleet operates in a complex, changing and uncertain environment. The strong specialization of the Belgian fleet with regard to fishing method (93% beam trawlers) and target species (mainly flatfish) makes the Belgian fisheries sector rather vulnerable to these continuously changing circumstances. Recently the sector is faced with yet another factor which contributes to the growing uncertainty: climate change. Climate change will most probably impose additional pressure on the sea fishery, but may also offer opportunities.

Comparison of mechanical disturbance in soft sediments due to tickler-chain SumWing trawl vs. electro-fitted PulseWing trawl

&NA; Tickler‐chain SumWing and electrode‐fitted PulseWing trawls were compared to assess seabed impacts. Multi‐beam echo sounder (MBES) bathymetry confirmed that the SumWing trawl tracks were consistently and uniformly deepened to 1.5 cm depth in contrast to 0.7 cm following PulseWing trawling. MBES backscatter strength analysis showed that SumWing trawls (3.11 dB) flattened seabed roughness significantly more than PulseWing trawls (2.37 dB). Sediment Profile Imagery (SPI) showed that SumWing trawls (mean, SD) homogenised the sediment deeper (3.4 cm, 0.9 cm) and removed more of the oxidised layer than PulseWing trawls (1 cm, 0.8 cm). The reduced PulseWing trawling impacts allowed a faster re‐establishment of the oxidised layer and micro‐topography. Particle size analysis suggested that SumWing trawls injected finer particles into the deeper sediment layers (˜4 cm depth), while PulseWing trawling only caused coarsening of the top layers (winnowing effect). Total penetration depth (mean, SD) of the SumWing trawls (4.1 cm, 0.9 cm) and PulseWing trawls (1.8 cm, 0.8 cm) was estimated by the depth of the disturbance layer and the layer of mobilized sediment (SumWing = 0.7 cm; PulseWing trawl = 0.8 cm). PulseWing trawls reduced most of the mechanical seabed impacts compared to SumWing trawls for this substrate and area characteristics.