Commercial Gillnet Research Articles

Disappearing fish: Grey seal depredation in a Baltic net fishery

Grey seals (Halichoerus grypus) in the Baltic Sea commonly attempt to steal fish captured in net fisheries. Seal depredation reduces economic yields for small-scale net fishers in a region already highly impacted by the collapse of important fish stocks. Visible damages in looted nets consist of injured fish with bite marks, or holes in the gear, but additional losses from seals eating entire fish without leaving a trace can often remain unnoticed. In this study, we estimated the daily rates of injured and of stolen fish in a commercial gillnet fishery operating off coast of Bornholm in the Central Baltic Sea in 2021–2022. Our results show that the Atlantic cod (Gadus morhua) is generally preferred over other target species [here, European plaice (Pleuronectes platessa), European flounder (Platichthys flesus), and turbot (Scophthalmus maximus)]. Although the observed ratio of 5.65 cods disappearing for each cod retrieved with seal bite marks is similar to a study from the 2000’s, we show that average daily cod disappearance rates are linked to the duration a gear is left soaking in the same location.

Spatial Dynamics of Demersal Fish Assemblages off the Korean Coast in the East Sea.

This study assessed variations in demersal fish assemblages with respect to the study site and water depth. Seasonal samplings from May 2018 to March 2019 were conducted along the northern (Ayajin, Goseong) and southern (Hupo, Uljin) sites of the East Sea off the Korean coast, using commercial gill nets. Samples were collected at depths of ~50, ~80, ~150 m across the study sites, with concurrent monitoring of water column structures. A total of 73 species and 6250 specimens were collected. Distinctive fish species compositions were observed according to the study site and depth. Although Glyptocephalus stelleri was the most abundant fish species in both Ayajin and Hupo, Gadus macrocephalus, Icelus cataphractus, and Alcichthys elongatus were most predominant in Ayajin, whereas Cleisthenes pinetorum, Hippoglossoides dubius, and Gymnocanthus herzensteini were more prevalent in Hupo. In terms of depth layer, in Ayajin, G. stelleri dominated in both intermediate and deeper layers, with Hemilepidotus gilberti, A. elongatus, Enophrys diceraus common in shallower depths. Conversely, in Hupo, G. stelleri, C. pinetorum, and A. nadeshnyi dominated across all depth layers, whereas Dasycottus setiger and G. herzensteini dominated in deeper and shallower depths, respectively. Significant influences of the study site and water depth on fish assemblage structures were observed due to variations in water temperature at the seasonal thermocline boundary.

What bycatch tells us about the diet of harbor and gray seals and overlap with commercial fishermen

Northwest Atlantic harbor (Phoca vitulina vitulina) and gray (Halichoerus grypus atlantica) seal populations are recovering from early to mid-20th century exploitation, increasing their biological interactions and bycatch in Northeastern US commercial fisheries. We evaluated the seals’ diet composition and compared their prey to commercial catches to assess trophic overlap and potential competition with commercial fisherman target catches. We obtained 148 harbor and 178 gray seal stomach samples from bycatch events that occurred between 2004 and 2018. We learned from the hard part remains that the majority of seals bycaught are young-of-the-year (≤12 months old) that consume a wide breadth of prey across three trophic groups. There was a general dichotomy in extrinsic factors associated with seal diet in which 45% trophic niche separation was explained by non-overlapping harbor and gray seal phenology and pup haul-out locations that are adjacent to active fishing areas. Prey size estimated from fish otoliths and squid beaks recovered from stomach contents showed that gray seals consumed larger prey than harbor seals and prey sizes from both seals showed limited overlap with prey sizes caught by commercial gillnet fishermen. The most important prey to both seals included large (>20 cm) and small (≤20 cm) silver hake (Merluccius bilinearis), (≤40 cm) red hake (Urophycis chuss), gulf stream flounder (Citharichthys arctifrons), medium (21–40 cm) white hake (Urophycis tenuis), and (<50 cm) Atlantic cod (Gadus morhua). Important prey to harbor seals that did not overlap with gray seals were Acadian redfish (Sebastes fasciatus), Atlantic herring (Clupea harengus), longfin (Doryteuthis pealeii), and shortfin squid (Illex illecebrosus). They contrasted with prey important to gray seals that did not overlap with harbor seals: yellowtail flounder (Limanda ferruginea), sand lance (Ammodytes spp.), Urophycis spp., and fourspot flounder (Hippoglossina oblonga). Despite the potential bias associated with opportunistic bycatch sampling, this study demonstrates the importance and value of utilizing carcasses retained from bycatch events, is complimentary to newer methodologies (i.e., DNA meta-barcoding), and fills data gaps in our understanding of the role recovering harbor and gray seal populations have on Northeastern US regional food webs.

Identifying priority areas using a multispecies approach for the conservation of marine megafauna species vulnerable to bycatch in commercial gillnet fisheries

Identifying priority areas using a multispecies approach for the conservation of marine megafauna species vulnerable to bycatch in commercial gillnet fisheries

Assessment of motorized commercial gillnet fishery of the three commercially important fishes in Lake Tana, Ethiopia

This study was conducted from July 2010 to June 2011 to assess the status of motorized commercial gillnet fishery of the three commercially most important fishes in Lake TanaLabeobarbusspp., Oreochromisniloticus(Nile tilapia) and Clariasgariepinus (African catfish). Catches were monitored daily and additional information on number of gillnets used, mesh size of gill nets and location of fishing grounds were collected. Data on the Labeobarbus species composition were collected 3 days per month. The total fish yield from the main fishing grounds of the commercial fishery was 238 metric tons/year. The catch of tilapia was 71%, that of catfish 18% and that of Labeobarbus 11% of the total catch. Peak production was from March - May 2011. Most effort (80%) was allocated to the North Eastern Floodplain (NEFP). The total number of gillnets set and boat trips made were 85,943 and 3104, respectively, which is much higher than reported previously. Consequently, the Catch per Unit effort (CpUE; kg/trip) of the 2010/2011 production year was with 64.7 ± 3 (95% CL) kg much lower than reported previously. The CpUE of all three fish taxa was lower than previously reported, but especially the LabeobarbusCpUE was much lower than before (6.1 kg). An integrated management plan and research program for the Lake Tana basin is urgently required to prevent the collapse of an important fishery and the extinction of the only known cyprinid species flock in the world.

Перспективи промислового використання карася Київського водосховища

Purpose. Establishing modern parameters of Prussian carp population as a promising object for increasing commercial fish productivity of the Kyiv Reservoir. Methodology. The work is based on field materials obtained during the period 2019-2021 on the Kyiv Reservoir. Data collection and processing was carried out in accordance with generally accepted methods. Ichthyological material was collected from catches of the survey gill nets with mesh sizes of 30-120 mm and commercial gill nets with mesh sizes of 38-50 and 75-100 mm. A total of 1215 net-days of surevy and commercial nets were analyzed during the study period, of which 2372 specimens of Prussian carp were analyzed. Statistical data processing was carried out using MS Excel. Findings. The main parameters that characterize the commercial stock of Prussian carp of the Kyiv Reservoir in the context of optimizing the commercial fishing pressure on this species have been determined. It was found that the state of recruitment of the commercial stock of this species was satisfactory and the dynamics of its size-age structure indicated a low commercial fishing pressure, especially on the right wing of the variation series — the average annual fishing mortality in 2019-21 was φM=0.13. The majority of the commercial stock is steadily composed of age-6 to age-10 fish, the share of older age groups was consistently high, which led to fluctuations in the weighted average age in catches within 6.7...8.5 years. It was shown that the intensification of Prussian carp harvest (due to specialized fishing with nets with mesh size of 50 mm) would allow increasing catches in the Kyiv Reservoir by 52...65%. Originality. For the first time, a detailed description of Prussian carp of the Kyiv Reservoir is presented as an adventive species that quickly assimilates free econiches and requires scientifically based measures to localize its abundance. Practical value. Based on the processing of information on the state of stocks of Prussian carp as part of the source of commercial fishery of the Kyiv Reservoir, the procedure of intensification of its commercial exploitation was determined, taking into account both fishery and environmental aspects. Proposals regarding the quantitative characteristics of specialized fishery with nets with mesh sizes of 50-60 mm are included in the new edition of the Rules for commercial fishery in inland fisheries water bodies (parts thereof). Key words: Kyiv Reservoir, Prussian carp, size-age structure of the catch, commercial stock, specialized fishing.

Management of gillnet bycatch of cetaceans in New Zealand

Bycatch of dolphins in inshore gillnets first attracted scientific and management attention in New Zealand in the 1980s. During 1984-88, 50-150 dusky dolphins were killed each year at Kaikoura in gillnets set at the surface to catch bait for rock lobster. At the same time, annual catches of 20-100 Hector’s dolphins occurred in Canterbury waters in bottom-set commercial and recreational gillnets. These catches resulted in the banning of surface-set gillnetting at Kaikoura in 1989 and in the creation of the Banks Peninsula Marine Mammal Sanctuary in 1988 to protect Hector’s dolphin. An additional gillnet closure was established to protect North Island Hector’s dolphin in 2003. A key problem is that current information on catches in these and other areas is scant. One observer programme has been successfully implemented in a commercial gillnet fishery (Canterbury area, 1997/98 fishing season). Its estimate of Hector’s dolphin bycatch (17) is clearly unsustainable by the local population. Pingers have been voluntarily used in these fisheries, but there are no data establishing their effectiveness, and it has not been possible to ensure consistency of pinger use. There are no reliable estimates of numbers taken in recreational fisheries. Area closures are used to mitigate gillnet bycatch of Hector’s dolphin, however it appears that the Banks Peninsula Marine Mammal Sanctuary is not large enough to ensure the persistence of the Canterbury population. There is a bycatch limit in place for this population, although it is unenforced. We argue that management of this species via bycatch limits is not practical, however. Hector’s dolphin’s low abundance and separation into several distinct populations means that appropriate bycatch limits would be very small, and this necessitates very comprehensive observer coverage to be confident they are not exceeded. We propose that increasing the size of protected areas is the most reliable option for conservation.

The significance of the Sikunga Fish Protected Area towards fisheries conservation in the Zambezi River, Namibia

AbstractLarge cichlids of the genera Oreochromis, Tilapia, Serranochromis and Sargochromis are the backbone of the gillnet fishery of the Zambezi River, but Oreochromis spp. is currently over‐exploited and at risk of local extinction. As a result, the Sikunga Fish Protected Area was established in 2012 to restore a decline in fish stocks of the Zambezi River, but its efficiency has never been assessed. The aim of this study was to compare fish assemblages, abundance and mean sizes between a fish protected area (Sikunga FPA) and nonprotected areas; the Sikunga Buffer Zone and the Lisikili Fished Area on the Zambezi River. Monthly gillnet surveys, using commercial gillnets, were conducted from July to December 2020. Overall catch rate (CPUE) by weight of all species differed significantly among the sampling sites (Kruskal–Wallis H‐test, range χ2(2) = −27.95, p = 0.0001). The highest CPUE was recorded at Sikunga Fish Protected Area (2.85 kg/set ± 0.42 (SE)), followed by the Sikunga Buffer Zone (0.93 kg/set ± 0.16 (SE)) and the Lisikili Fished Area (0.61 kg/set ± 0.14 (SE)). Further analyses showed that the mean sizes of O. andersonii, Coptodon rendalli, Serranochromis and macrocephalus were significantly larger at the Sikunga Fish Protected Area than at the Lisikili Fished Area. A particularly interesting observation pertains to a lack of variation in catch rates of Serranochromis macrocephalus and mean sizes of Clarias gariepinus between the protected and nonprotected areas, signifying that fish protected areas can be species‐specific, and not all species in a water body will respond positively to protection. The overall results of this study support the innovation of fish protected areas as a fundamental tool in attaining sustainable management of fishery resources in inland waters of Namibia and other developing countries.

Pingers reduce harbour porpoise bycatch in Norwegian gillnet fisheries, with little impact on day-to-day fishing operations

A field trial was conducted to determine the effect of acoustic deterrent devices (ADDs, or pingers) on harbour porpoise (Phocoena phocoena) and harbour seal (Phoca vitulina) bycatch in three Norwegian commercial gillnet fisheries targeting cod (Gadus morhua), saithe (Pollachius virens) and monkfish (Lophius piscatorius). Catch data on 3500 net-km-days were collected by 8 fishing vessels operating gillnets in high bycatch regions over two years. A total of 20 harbour porpoises and 9 harbour seals were bycaught, with 19 harbour porpoises and 6 harbour seals taken in control (non-pingered) nets. Bycatch was modelled using a generalized additive mixed modelling approach and fitted with penalized maximum likelihood. Modelling results indicated that using pingers on gillnets reduced the risk of bycatching a harbour porpoise by an estimated 94% (95% confidence interval CI 77–100%) compared to ordinary pinger-free nets. The effect of pingers was not significantly different between different fisheries. The pingers also had no significant effect on catch rates of fish (Wilcoxon rank sum test, p = 0.24) or harbour seals (Wilcoxon rank sum test, p = 0.19). Self-reported pinger-associated extra time costs on day-to-day fishing operations were low, averaging about 2.8 min per operation. These results add to a growing body of scientific evidence that pingers can lead to substantial reductions in harbour porpoise bycatch rates in gillnet fisheries, and that extra time costs associated with operating nets with pingers are low.

СУЧАСНИЙ СТАН ОСНОВНИХ ПРОМИСЛОВИХ ВИДІВ РИБ КРЕМЕНЧУЦЬКОГО ВОДОСХОВИЩА

The major quantitative and qualitative indicators of the main commercial fish species of the Kremenchug reservoir were analyzed in the study and the main parameters of the optimal commercial fisheries catch by gill nets were determined. Original materials for investigations were data of control commercial catches in the Kremenchug reservoir during 2021. Conservative techniques were used in sampling and analysis during ichthyological studies.
 Materials on the main commercial fish species in the Kremenchug reservoir were collected. Biological materials from commercial gill nets were selected and the fish productivity of the reservoir was determined.
 The work provides up-to-date information on the main biological indicators of commercial fish species of Kremenchug reservoir in modern conditions of anthropogenic press and calculates scientifically proven parameters of optimal commercial fish harvest.
 The article can be used by scientists, practitioners, fisheries patrol and the State Ecological Inspectorate to predict future catches and knowledge of the current situation with the main industrial fish species in the Kremenchug Reservoir. Modern material on the main commercial fish species will help to see the real picture in the reservoir and take necessary fish protection measures.

Aspects of the biology and population structure of Oreochromis niloticus, Coptodon zillii and Oreochromis leucostictus tilapia in Lake Naivasha, Kenya

AbstractThe Nile tilapia (Oreochromis niloticus), Redbelly tilapia (Coptodon zillii) and Blue spotted tilapia (Oreochromis leucostictus) were introduced into Lake Naivasha in the 1950s to diversify and boost the lake fisheries. These species have since been exploited in the commercial gillnet fishery. The fish stocks, however, are currently facing problems of pollution, wetland degradation and intense fishing pressures. Accordingly, the present study investigated some aspects of the biology and population structure of the three tilapiine species in light of changing lake conditions. Fish samples were collected from January to December 2019 using multifilament gillnets of 2.0‐to‐6.0‐inch mesh sizes. Immediately after their capture, the fish specimens were transported to the lab for morphometric measurements and analysis of maturity. The highest and lowest mean (±SD) total length (TL) were exhibited by O. niloticus (22.9 ± 5.2 cm) and C. zillii (14.5 ± 2.1 cm), respectively. Similarly, O. niloticus exhibited a higher mean weight (268.4 ± 18.0 g) and body depth (7.6 ± 2.5 cm), compared to the other species. O. niloticus exhibited an isometric growth pattern, while C. zillii and O. leucostictus exhibited negative allometric growth. The fish species were in good condition, with mean condition factors of 1.59 ± 0.07, 1.51 ± 0.06 and 1.23 ± 0.03 for C. zillii, O. leucostictus and O. niloticus, respectively. Furthermore, the condition factors for O. leucostictus and O. niloticus varied by size class, exhibiting a decreasing trend with increased fish size. The male:female sex ratio revealed significantly more males than females for O. niloticus (2.30:1.0) and O. leucostictus (2.36:1.0), although not for C. zillii (1.15:1.0). O. niloticus (28.0–29.0 cm TL) matured at a larger size, followed by O. leucostictus (21.0–26.0 cm TL), and C. zillii (15.0–17.0 cm TL). In comparison, values of length‐weight parameters, condition factor, sex ratio and size at first maturity of the studied fishes varied in Lake Naivasha, Victoria, Albert and George. The gillnet mesh ≤3.0″ mainly targeted small‐sized immature fish, while mesh ≥4″ targeted mature fish, particularly O. niloticus. It was concluded that gillnets with ≥4.0″ mesh could safely be applied for this fishery in Lake Naivasha, whereas the use of smaller mesh should be controlled to protect the juveniles as a means of maintaining sustainable fisheries. Furthermore, the present study also provided useful information, including size at maturity that is vital for setting mesh size for managing the lake fishery.

Conservation implications for post-fledging dispersal of yellow-eyed penguins/hoiho

With the extinction of yellow-eyed penguinsMegadyptes antipodeson mainland New Zealand predicted within the next few decades, identifying preventable causes of mortality during the juvenile dispersal period is critical. Between 2017 and 2019, we tracked 30 juvenile yellow-eyed penguins during their post-fledging dispersal to determine their dispersal trajectory, marine habitat use, and to identify overlap with commercial gillnet fishing operations, a known cause of yellow-eyed penguin mortality. Usingk-nearest neighbour local convex hulls, we identified core foraging hotspots (50% isopleths) and main marine habitats (95% isopleths). We identified environmental covariates that influenced penguin habitat use during separate stages of the dispersal journey. After fledging, juvenile yellow-eyed penguins from the Otago coast dispersed and foraged 140 to 692 km (mean ± SD = 359.9 ± 129.3 km) from their natal locations, north-north-east into the Canterbury Bight, a region where penguin bycatch in commercial gillnet fishing operations has been recorded. There was a 51.6% overlap between core juvenile foraging hotspots and known commercial gillnet fishery locations (from 2017 to 2019). Less than 12.6% of the core foraging hotspots occurred within the 4 nautical mile gillnet prohibition zone implemented in 2008. A recent extension in the size of the gillnet prohibition zone in 2020 increased this spatial overlap to cover 36.9% of core foraging hotspots and 28.9% of main marine habitats. These findings highlight the vulnerability of juvenile yellow-eyed penguins at this critical life-stage, but there is opportunity to amalgamate marine protection measures with other endangered species with overlapping distributions that are also at risk of gillnet-related mortality.

Impacts of trophy collection and commercial fisheries on sawfishes in Queensland, Australia

Globally, populations of sawfishes (Batoidea: Pristidae) have declined due to directed fishing and bycatch. Before international trade protection, sawfish rostra were often collected as trophies, and sawfish fins could fetch some of the highest prices in the shark fin trade. Australia is now considered a global stronghold for 4 out of 5 species. The present study analyses data of 723 sawfish rostra from all 4 Australian species that were caught in Queensland over the last 100 years. The majority of rostra (92.2%) originated from gillnet captures. Morphometric data from rostra were used to calculate sawfish total lengths and ages at mortality using published ratios. Rostra were split into the mass retention subgroup (n = 569), which originated from ≤5 active commercial inshore gillnet fishers with known large geographic origin regions and decades of origin, and the small-scale retention subgroup (n = 154), for which individual years and locations are known. Across all regions of Queensland, species composition changed significantly before and after the year 2000. After 2000, Anoxypristis cuspidata became the dominant species on the east coast. Large adult Pristis zijsron were present across Queensland before 2000 and numbers declined after this. The commercial gillnet industry likely impacts all life history stages of sawfishes. Since 2002, it has become compulsory for Queensland’s commercial fishers to report interactions with sawfishes in SOCI (Species of Conservation Interest) logbooks. Comparison of rostra from gillnet captures with logbook data indicates that underreporting is higher than previously assumed.

Using acoustically visible gillnets to reduce bycatch of a small cetacean: first pilot trials in a commercial fishery

Bycatch of protected species, particularly small cetaceans, in gillnets is a worldwide concern. One hypothesis for this is that echolocating cetaceans entangle because they do not perceive conventional gillnets as impenetrable barriers, owing to the gillnet’s faint echo. A gillnet modified for improved acoustical visibility was tested in a first pilot trial in a commercial gillnet fishery targeting turbot (Scophthalmus maeoticus) on the Turkish Black Sea coast. This study is the first demonstration of the viability of using a gillnet equipped with small acrylic glass spheres to reduce bycatch of harbor porpoises in a commercial fishery and provides the basis for full-scale sea trials of the gear in commercial fisheries through a power analysis. In these pilot experiments, the focus lied on the handling of the gear and identification of requirements for a full-scale trial, but results include promising bycatch data for an endangered echolocating marine mammal (Phocoena phocoena) and no reduction in catch efficiency of a bottom-dwelling, vulnerable species (Raja clavata).

Triennial migration and philopatry in the critically endangered soupfin shark Galeorhinus galeus

Abstract Globally, one quarter of shark and ray species are threatened with extinction due to overfishing. Effective conservation and management can facilitate population recoveries. However, these efforts depend on robust data on movement patterns and stock structure, which are lacking for many threatened species, including the Critically Endangered soupfin shark Galeorhinus galeus, a circumglobal coastal‐pelagic species. Using passive acoustic telemetry, we continuously tracked 34 mature female soupfin sharks, surgically implanted with coded acoustic transmitters, for 7 years via 337 underwater acoustic receivers stationed along the west coast of North America. These sharks and an additional six were also externally fitted with spaghetti identification tags. Our tagging site was a shallow rocky reef off La Jolla (San Diego County), California, USA, where adult females were observed to aggregate every summer. Tagged soupfin sharks were highly migratory along the west coast of North America, between Washington, USA and Baja California Sur, Mexico. However, every 3 years, they returned to waters off La Jolla, California, where they underwent gestation. This is the first conclusive evidence of triennial migration and philopatry (‘home‐loving’) in any animal, which is apparently driven by this species’ unusual triennial reproductive cycle. Females of other shark and ray species with triennial reproductive cycles are also likely to exhibit triennial cycles of migration and philopatry. At least six (15%) of our tagged soupfin sharks were killed in commercial gillnets in Mexico. Policy implications. Identifying multiennial migratory cycles in mature female sharks can reveal hidden stock structure in the form of discrete breeding cohorts, which are spatially and temporally segregated as they cycle through different reproductive phases. Accounting for this complexity may improve the performance of spatially structured stock assessment models, particularly when fishery removals are spatially heterogeneous, as well as inform the spatiotemporal design of fishery‐independent surveys. In the United States, the soupfin shark is neither actively managed nor recognized as a Highly Migratory Species; however, given the highly migratory behaviour we report, this designation should be revisited by the US Pacific Fishery Management Council. Finally, given the extensive fishery removals in Mexico, any future management must be internationally cooperative.

Detection of fish movement patterns across management unit boundaries using age-structured Bayesian hierarchical models with tag-recovery data.

Tagging studies have been widely conducted to investigate the movement pattern of wild fish populations. In this study, we present a set of length-based, age-structured Bayesian hierarchical models to explore variabilities and uncertainties in modeling tag-recovery data. These models fully incorporate uncertainties in age classifications of tagged fish based on length and uncertainties in estimated population structure. Results of a tagging experiment conducted by the Ontario Ministry of Natural Resources and Forestry (OMNRF) on yellow perch in Lake Erie was analyzed as a case study. A total of 13,694 yellow perch were tagged with PIT tags from 2009 to 2015; 322 of these were recaptured in the Ontario commercial gillnet fishery and recorded by OMNRF personnel. Different movement configurations modeling the tag-recovery data were compared, and all configurations revealed that yellow perch individuals in the western basin (MU1) exhibited relatively strong site fidelity, and individuals from the central basin (MU2 and MU3) moved within this basin, but their movements to the western basin (MU1) appeared small. Model with random effects of year and age on movement had the best performance, indicating variations in movement of yellow perch across the lake among years and age classes. This kind of model is applicable to other tagging studies to explore temporal and age-class variations while incorporating uncertainties in age classification.

Assessing the impact of fisheries-related mortality of harbour porpoise (Phocoena phocoena) caused by incidental bycatch in the dynamic Norwegian gillnet fisheries

AbstractHarbour porpoise (Phocoena phocoena) bycatch for Norwegian commercial gillnet fisheries from 2006 to 2018 was estimated using a traditional ratio estimator and generalized additive linear mixed models, with weight of fish landed and number of gillnet hauls as proxies for fishing effort. Estimates were derived from data collected with a contracted reference fleet of small coastal vessels and scaled up to the whole fleet using data from landing statistics. Bycatch estimates exhibited large yearly variations, ranging from 1151 to 6144 porpoises per year. Bycatch estimates in 4 of the last 5 years were significantly less than in the preceding 2 years. The best ratio-based and model-based yearly bycatch estimates were 1580 porpoises [coefficient of variation, (C.V.) 0.10, 95% confidence interval (CI) 1302–1902] and 1642 porpoises (C.V. 0.15, 95% CI 1165–2142), respectively. About 75% of bycaught porpoises were taken in the cod (Gadus morhua) and monkfish (Lophius piscatorius) fisheries, while the rest were taken in a variety of different gillnet fisheries. Our results suggest that bycatch of harbour porpoise in Norwegian gillnet fisheries has been unsustainable for several of the last 13 years but are currently within international bycatch limits due to a recent reduction in monkfish fishing effort.

Synthetic harbour porpoise (Phocoena phocoena) communication signals emitted by acoustic alerting device (Porpoise ALert, PAL) significantly reduce their bycatch in western Baltic gillnet fisheries

Gillnet fisheries are one of the main anthropogenic causes of harbour porpoise (Phocoena phocoena L., 1758) mortality in the Baltic Sea. A new kind of acoustic alerting device (Porpoise ALert, PAL) was tested in commercial gillnet fisheries in the western Baltic. PAL emits 133 kHz synthetic harbour porpoise communication signals, unlike conventional acoustic deterrent devices (pingers), which emit artificial noise. Trials were undertaken by three commercial gillnet vessels conducting 778 trips during standard fishing operations from 2014 to 2016. In all, 1120 PAL-equipped net strings were tested against 1529 simultaneously set control strings with no devices. We tested two versions of the PAL (v1 and v2) consecutively. These were spaced <=210 m apart on the gillnet floatlines, with all devices pointing in the same direction to ensure complete acoustic coverage of the strings. Two vessels fished in Kiel Bight and around Fehmarn Island in German waters, and the third vessel fished in the Øresund, in inner Danish waters. Overall, 18 harbour porpoises were bycaught in control strings (mean 0.01 ± 0.1/haul), and five harbour porpoises were taken as bycatch in strings equipped with PALs (0.004 ± 0.07/haul). The number of net string bycatches was analysed using a generalised linear mixed model (GLMM). The model applied to all observations revealed that the expected bycatch was significantly influenced by PAL deployment (p < 0.05), decreasing the expected bycatch by 64.9 % (95 % confidence interval (CI): 8.7–88.7 %). PAL effectiveness was also increased by reducing device spacing to <=200 m (16 bycatches in control, three in PAL strings; mean bycatch reduction 79.7 %). Additional model cases were applied to the data and are discussed. We conclude that, with this specific communication signal, PAL can significantly reduce harbour porpoise bycatch in gillnets deployed in the western Baltic Sea, thus reconciling anthropogenic activities with protection of the marine environment.

Gillnet fishers’ knowledge reveals seasonality in depth and habitat use of cod (Gadus morhua) in the Western Baltic Sea

AbstractPractical and applied knowledge of local fishers can help to improve our understanding of target species ecology and fisheries management decisions. In the Western Baltic Sea (WBS), the spatio-temporal distribution of cod is still largely unknown despite decades of research. We studied changes in cod distribution by obtaining information on temporal depth and habitat use of cod from commercial gillnet fishers using semi-directed interviews supplemented by at-sea observer data. Linear and non-linear regression analyses revealed significant relationships between depth use of cod and sea surface temperature (SST) as well as thermal stratification. Moreover, habitat use was related to SST and residence depth of cod. Areas deeper than 15 m were favoured from late December until March during low SST and a mixed water column (spawning) and also from July until August during high SST and strong thermal stratification (summer aestivation). Shallower areas were favoured during the rest of the year. The depth and habitat use displayed distinct seasonal up- and downslope movements of cod. This study highlights the importance of shallow-water and structured habitats for cod in the WBS and the value of local knowledge held by fishers for a better understanding of the distributional dynamics of important marine resource populations.

Stock Structure Analysis of the Japanese Spanish Mackerel Scomberomorus niphonius (Cuvier, 1832) Along the China Coast Based on Truss Network

The Japanese Spanish mackerel Scomberomorus niphonius (Cuvier, 1832) is widely distributed in the subtropical and temperate waters of the northwestern Pacific Ocean, supporting one of the most important commercial fisheries in China. However, ignoring the potential population structure changes induced by fishing pressure and climate change may undermine the population stability under the current management strategy. In this study, the population structure of the Japanese Spanish mackerel was investigated based on a morphometric truss network system. A total of 534 individuals were randomly collected from commercial gill nets spanning eight major spawning grounds in the Bohai, Yellow, and East China Seas during the peak spawning seasons respectively. A total of 17 measurements (including eye diameter) were conducted in each specimen and subjected to principal component analysis (PCA) and discriminant function analysis (DFA). The results of PCA indicated that the first two factors cumulatively caused 78.38% of the total morphometric variation and observable differences, primarily fin the caudal and trunk areas. The results of DFA revealed that the eight spawning groups can be divided into three stocks, i.e. , southern, middle, and northern stocks, with 68.7% of total accuracy. In contrast to previous studies, the spawning groups of the Japanese Spanish mackerel demonstrated a tendency to disperse to northern regions. In conclusion, this study found that to maintain the stability of the population structure and the total production of Japanese Spanish mackerel S. niphonius (Cuvier, 1832), a newly revised management method should be developed and implemented.