Petri Suuronen

Mortality of herring escaping from pelagic trawl codends

Mortality of Baltic herring (Clupea harengus L.) escaping through the meshes of 26 mm and 36 mm diamond mesh (stretched mesh length) codends attached to a pelagic trawl was studied. After 30 min of towing, a cage that encircled the codend (and therefore captured codend escapees) was released from the trawl and anchored at depths of 7–17 m for periods of 1.5–9 days. Fish escaping through the end of an open codend extension were also collected and held in the same manner as fish that escaped through the codend meshes. No difference between mortality of herring escapees from 26 mm and 36 mm codends was observed, and herring that escaped through the open codend extension suffered the same level of mortality as fish that passed through codend meshes. Significant differences in mortality of escapees were observed between small (<12 cm) and large (12–17 cm) body size categories (P ≤ 0.05). For all three codend types combined, the 7-day post-capture mortality estimate for small herring was 72% (95% confidence intervals = 47–88%), whereas the estimate for large herring was 30% (95% confidence intervals = 11–60%). The predicted 14-day post-capture mortalities were 91% and 62% for small and large escapees, respectively. Mortality of 12–17 cm herring captured by seine-net and handline (i.e. controls) and held 9–16 days in cages (9–13%) was substantially lower than that of trawl escapees. Codend escapees sustained more skin damage than herring caught by seine and handline. Liver glycogen stores of small (7–11 cm) codend escapees were nearly depleted during the first 2 days after capture, and remained low during the 5-day observation period. In contrast, seine-caught herring showed significantly higher and increasing glycogen content after 3 days of recovery. Results of this study indicate that factor(s) other than passing through codend meshes caused most of the observed mortality for herring escapees. We suggest that skin injuries and exhaustion that occur while fish are inside the funnelling rear part and the codend of the trawl are the most likely causes of escapee mortality. On the basis of these results, the justification and usefulness of codend mesh size management in the herring trawl fishery is questionable.

Causes and methods to estimate cryptic sources of fishing mortality.

Cryptic, not readily detectable, components of fishing mortality are not routinely accounted for in fisheries management because of a lack of adequate data, and for some components, a lack of accurate estimation methods. Cryptic fishing mortalities can cause adverse ecological effects, are a source of wastage, reduce the sustainability of fishery resources and, when unaccounted for, can cause errors in stock assessments and population models. Sources of cryptic fishing mortality are (1) pre-catch losses, where catch dies from the fishing operation but is not brought onboard when the gear is retrieved, (2) ghost-fishing mortality by fishing gear that was abandoned, lost or discarded, (3) post-release mortality of catch that is retrieved and then released alive but later dies as a result of stress and injury sustained from the fishing interaction, (4) collateral mortalities indirectly caused by various ecological effects of fishing and (5) losses due to synergistic effects of multiple interacting sources of stress and injury from fishing operations, or from cumulative stress and injury caused by repeated sub-lethal interactions with fishing operations. To fill a gap in international guidance on best practices, causes and methods for estimating each component of cryptic fishing mortality are described, and considerations for their effective application are identified. Research priorities to fill gaps in understanding the causes and estimating cryptic mortality are highlighted.

Avoidance and escape behaviour by herring encountering midwater trawls

The main objective of this study was to assess how and to what extent Baltic herring avoid and escape midwater trawls under commercial fishing conditions. Information was obtained by acoustic, visual and direct sampling techniques. The majority of herring schools in the path of an approaching trawl were observed to enter the trawl. When they did avoid the trawl it was by swimming downwards. The frequency of strong avoidance reactions was significantly higher during daytime than at night. Herring entering the trawl passed quickly into the funnelling rear part of the trawl, where they swam vigorously, trying to maintain their position in relation to the trawl. Fish often struck against the netting and their scales were seen passing through the meshes. Some herring escaped through the meshes of the upper rear panels of the trawl body, especially in the dark. Escaping herring were significantly smaller than those caught in the codends. Nevertheless, most herring ended up in the codend. We conclude that, although herring occasionally avoid and escape midwater trawls, the majority of fish in the trawl path are caught in the codend.

Effects of catching method on different quality parameters of Baltic herring (Clupea harengus L.)

Abstract The effects of three catching methods; trawling, gillnetting and poundnetting, on the quality of Baltic herring when stored at 8–16 °C were studied. The proportion of dead fish in the catch increased when the trawling time was increased from 2 to 5 h. Rigor mortis appeared soonest in fish caught by the gillnetting method being practiced. The inosine monophosphate (IMP) content was also lowest in gillnet-caught fish, indicating a loss of freshness. Differences between fish caught by the different methods could not be detected by sensory testing or on the basis of the dielectric properties of fish as measured by a Torrymeter GR device.

Suitability of motorized under-ice seining in selective mass removal of coarse fish

The efficiency of modern, fully motorized under-ice winter seining technique was studied in the selective mass removal of coarse fish in the eutrophicated Lake Vesijarvi, southern Finland in 1992–1994. The total catch in 57 seine hauls was 69000 kg and the average catch per haul 1210 kg (250–6500 kg). The average biomass removed from the fishing area per haul was 120 kg ha−1 (SD 116) in the deepest basins (depth > 15 m), where smelt (Osmerus eperlanus L.) dominated the catches. The corresponding catch was 192 kg ha−1 (SD 187) in a shallower basin (< 15 m) where the catch comprised mainly cyprinids, all age classes of bream (Abramis brama L.) and bleak (Alburnus alburnus L.) and juveniles (1 + and 2+) of roach (Rutilus rutilus L.) and bleak. The catches were composed of aggregated fish since there was no correlation between the length of the haul and weight of the catch and the biomass removed from the fishing area was high. The under-ice pelagic schooling pattern of fish distribution was confirmed by scanning sonar monitoring which was effectively utilized in fishing. Winter seining is a suitable method for selective fish removal both in fisheries management and in biomanipulation of temperate lakes which are ice-covered for several months during a year.

Selectivity of flexible size-sorting grid in Mediterranean multispecies trawl fishery

The demersal multispecies trawl fishery in the western Mediterranean Sea has a poor selectivity; large numbers of juvenile fish are caught when using the legislated 40-mm diamond mesh cod end. The selectivity of a flexible sorting grid with 20-mm bar spacing (BS20) installed in front the conventional trawl cod end was investigated. The standard covered cod end method was used. Data was adequate for analyzing the selectivity of European hake Merluccius merluccius, poor cod Trisopterus minutus, greater forkbeard Phycis blennoides and Norway lobster Nephrops norvegicus. The selectivity of the 20-mm sorting grid was compared with the performance of 40-mm diamond (DM40) and square mesh (SM40) cod ends. The effect of a guiding funnel on the performance of the grid (BS20-f) was also explored. Finally, the potential changes in yield per recruit (Y/R) and biomass per recruit (B/R) after implementing a sorting grid were explored. An overall improvement in the 50% selection length (L50) with all four species was substantial when comparing the BS20 to the DM40. Higher improvement in L50s was achieved when the grid was equipped with the guiding funnel. When comparing the performance of the BS20-f to the SM40, there was no marked difference in L50s. The Y/R and B/R, however, are substantially higher with BS20-f for poor cod and about the same for European hake. BS20 compared to SM40 achieved lower Y/R and B/R for all four species; the smallest difference was observed for Norway lobster. Further development is required if the sorting grid is to be introduced into commercial fisheries.

An improved method for studying survival of fish that escape through meshes of trawl codends

Abstract The objective of this study is to develop and test a practical field method for measuring survival of fish that pass through trawl cod-end meshes under commercial fishing conditions. This method permits the collection of escaping fish during any moment of a haul. Hence, survival can be assessed for short and long tows, small and large catch quantities, and for any desired depth using this technique. The method involves the use of a cage (with front and rear gates) that is attached to the terminal end of a cod-end cover. The front and rear gates can be held open during the tow, allowing cod-end escapees to pass freely through the cage and into the open sea. The rear gate is closed using a pre-set timer when a sample of escapees is needed. After a suitable sampling duration, pre-set timers activate the closure of the front gate and trigger the release of the cage from the cover. The cage is then left on the sea bed at the place of release for subsequent monitoring of fish condition (e.g., by divers). Trials were conducted on a demersal trawl vessel in the Baltic Sea in 1996–1997. We found that the method performed well under all circumstances for Baltic cod ( Gadus morhua L.) that escaped through trawl cod-end meshes. The method itself was not detrimental to the escapees.

Ecological data from observer programmes underpin ecosystem-based fisheries management

Ecological data from observer programmes underpin ecosystem-based fisheries management Eric Gilman*, Mariska Weijerman, and Petri Suuronen Hawaii Pacific University, Pelagic Ecosystems Research Group, 3661 Loulu Street, Honolulu, HI, USA Pacific Islands Fisheries Science Center, NOAA Fisheries, 1845 Wasp Boulevard, Building 176, Honolulu, HI, USA Fisheries and Aquaculture Department, Food and Agriculture Organization of the United Nations (FAO), Viale delle Terme di Caracalla, Rome, Italy *Corresponding author: tel: þ1.808.888.9440; e-mail: fisheriesresearchgroup@gmail.com

The effect of lunar cycle, tidal condition and wind direction on the catches and profitability of Japanese common squid Todarodes pacificus jigging and trap-net fishing

Jigging with artificial lights (squid jigging) and deploying of large scale trap-net (also known as a set-net in Japan), are the major methods to capture Japanese common squid Todarodes pacificus in western Japan. Squid jigging is a highly selective fishing method. However, it consumes large amount of energy for steaming to the fishing ground and for lighting. In contrast, trap-net fishing requires substantially less energy but its capture efficiency is strongly influenced by its stationary mode of capture. The primary objective of this study was to analyze how various environmental and biological factors such as the lunar cycle, tidal condition, wind direction and squid abundance affect the capture efficiency of squid jigging and trap-net fishing. We analyzed the effect of these factors on squid catch in five Fisheries Cooperative Associations located on four islands in Nagasaki Prefecture, western Japan. Our analysis shows that squid catch in jigging and trap-net fishing is mainly influenced by the lunar cycle but also tide and wind direction play a marked role. In addition, squid abundance significantly affects the catches in trap-net fishing. Recommendations are made to improve the overall profitability of squid fishing by proper choice of the capture method, location and season.

Bottom trawl fishing footprints on the world’s continental shelves

Significance We conducted a systematic, high-resolution analysis of bottom trawl fishing footprints for 24 regions on continental shelves and slopes of five continents and New Zealand. The proportion of seabed trawled varied >200-fold among regions (from 0.4 to 80.7% of area to a depth of 1,000 m). Within 18 regions, more than two-thirds of seabed area remained untrawled during study periods of 2–6 years. Relationships between metrics of total trawling activity and footprint were strong and positive, providing a method to estimate trawling footprints for regions where high-resolution data are not available. Trawling footprints were generally smaller in regions where fisheries met targets for exploitation rates, implying collateral environmental benefits of effective fisheries management. Bottom trawlers land around 19 million tons of fish and invertebrates annually, almost one-quarter of wild marine landings. The extent of bottom trawling footprint (seabed area trawled at least once in a specified region and time period) is often contested but poorly described. We quantify footprints using high-resolution satellite vessel monitoring system (VMS) and logbook data on 24 continental shelves and slopes to 1,000-m depth over at least 2 years. Trawling footprint varied markedly among regions: from <10% of seabed area in Australian and New Zealand waters, the Aleutian Islands, East Bering Sea, South Chile, and Gulf of Alaska to >50% in some European seas. Overall, 14% of the 7.8 million-km2 study area was trawled, and 86% was not trawled. Trawling activity was aggregated; the most intensively trawled areas accounting for 90% of activity comprised 77% of footprint on average. Regional swept area ratio (SAR; ratio of total swept area trawled annually to total area of region, a metric of trawling intensity) and footprint area were related, providing an approach to estimate regional trawling footprints when high-resolution spatial data are unavailable. If SAR was ≤0.1, as in 8 of 24 regions, there was >95% probability that >90% of seabed was not trawled. If SAR was 7.9, equal to the highest SAR recorded, there was >95% probability that >70% of seabed was trawled. Footprints were smaller and SAR was ≤0.25 in regions where fishing rates consistently met international sustainability benchmarks for fish stocks, implying collateral environmental benefits from sustainable fishing.