Gillnet Fisheries Research Articles

Per-recruit analysis and proposed sustainable harvest strategy for rednose labeo Labeo rosae and Mosambique tilapia Oreochromis mossambicus in Flag Boshielo Dam, Olifants River, South Africa

Inland fisheries development in South Africa has been proposed to improve rural livelihoods and food security. However, inland fisheries are susceptible to collapse through overfishing. Therefore, establishing biological reference points for these fisheries is essential for long-term sustainability. Here, spawner-biomass-per-recruit (SBR) analysis was used to set biological reference points for a gillnet fishery targeting the rednose labeo Labeo rosae and Mozambique tilapia Oreochromis mossambicus at Flag Boshielo Dam on the Oliphants River in Limpopo Province, South Africa. Both species are currently exploited at the impoundment by subsistence fishers that keep their entire catch. The current yields are estimated at 59 and 172 g yr−1 for L. rosae and O. mossambicus, respectively. The per-recruit analysis showed that age-at-first-capture for both species should be 4 years, at minimum lengths of 280 mm TL and 250 mm TL for L. rosae and O. mossambicus, respectively, with fishers using a minimum mesh size of 100 mm to reduce the risk of stock collapse. To maintain a SBR value greater than 25% of a pristine population, the L. rosae yield could be increased to 98 g yr−1, while the O. mossambicus yield could be increased to 176 g yr−1, following implementation of the recommended mesh size and size limits. A per-recruit analysis for O. mossambicus that includes recruitment variability and error to accurately calculate the biological reference points is recommended.

Fishers' perception and activity shifts in a dolphin bycatch mitigation context

Fishing exclusion zones aim to reduce cetacean bycatch, but often neglect social and ecological side effects, exacerbating social injustices and reshaping fishing practices. We present a case study of an endangered population of Lahille's bottlenose dolphins (Tursiops truncatus gephyreus) in a Brazilian coastal lagoon, known for its unique cooperation with net-casting fishers. However, the local small-scale fisheries (SSF) also employ other fishing gears that contribute to dolphin bycatch, prompting the enforcement of a fishing ban in the area. We investigated fishers' socioeconomic conditions, evidence of social disparities, perceptions on dolphin bycatch, and projected changes on fisheries activities to understand the repercussions of the ban. We categorized 128 surveyed fishers into five groups based on socioeconomic factors and their reliance on dolphin-cooperative and gillnet fisheries. Our findings revealed significant disparities in social capital among fishers and their different attitudes towards dolphin bycatch, highlighting unforeseen consequences. To investigate how fishers would likely shift fishing practices, we considered two management scenarios: 1) should dolphin-fishing cease due to unsuccessful bycatch mitigation, fishing efforts would shift towards shrimp trawling, seine nets, and gillnet effort; 2) if gillnet-fishing is successfully banned as a bycatch mitigation measure, a shift to shrimp trawling and cast net effort. Both scenarios would intensify pressure on shrimp stocks, which are already heavily exploited in this socioecological system. Our research underscores the importance of contemplating alternatives to achieve sustainable outcomes when designing bycatch mitigation measure. Furthermore, our results point to the importance of a participatory governance approach, grounded in the socioeconomic context, to safeguard the livelihoods of small-scale fishers and foster broader conservation efforts beyond species-specific concerns.

Assessing the vulnerability of Elasmobranch species in the Bay of Bengal: Insights from Lakkha gill net fishery of Bangladesh

The elasmobranch population is declining in the Bay of Bengal of Bangladesh due to large-mesh gill net fishing, locally known as the Lakkha net, which primarily targets Indian threadfin (Leptomelanosoma indicum). This study was the first attempt to identify megafaunal bycatch in Lakkha fishing and assess its vulnerability using Productivity Susceptibility Analysis. A total of 40 elasmobranch bycatch species were identified, with sharks comprising 13 species from three families, while 27 rays belonged to six families, with the majority belonging to the Myliobatiformes order (60 %). Productivity and susceptibility scores were assigned to all identified species, with values ranging from 1.27 to 2.73 and 1.50 to 2.63, respectively. The target Lakkha fish exhibited the highest susceptibility score, followed by several pelagic sharks and eagle rays. Vulnerability assessment revealed that 31.7 % (n = 13) of species were highly vulnerable, while 43.9 % (n = 18) were classified as moderate, and 24.4 % (n = 10) were considered to have low vulnerability. All the high-risk megafauna species (n = 13) are classified as threatened by the global IUCN Red List. Sensitivity analysis highlighted susceptibility as a major contributor to species’ vulnerability. Alterations in susceptibility scores led to significant changes in the vulnerability status of many species. The overall data quality assessment indicated moderate data quality across species, with variability observed between productivity (76 % of species received a poor data quality score) and susceptibility attributes. However, vulnerability of these species can be reduced through adequate gear modification, shorter net deployment periods, adoption of safe discharge techniques, identification of critical habitats, and establishment of marine protected areas within this region. This study provides valuable insights into the species composition and vulnerability of elasmobranchs in the Lakkha gill net fishery, emphasizing the need for conservation measures to mitigate bycatch impacts on threatened species.

Can operational tactics compensate for weaker tensile properties of biodegradable gillnets?

This study aimed at discriminating between the effects of physical strain due to the interactive effect of gear operation and degradation, and degradation only, on the differences in breaking strength, strain, and stiffness at break of polybutylene succinate-co-adipate-co-terephthalate (PBSAT) (biodegradable) and PA6 (nylon) gillnet materials, and to compare the effect of fish species and resulting capture mode on catch efficiency for a roundfish species, Atlantic cod ( Gadus morhua), and a flatfish species, European plaice ( Pleuronectes platessa) in the Danish coastal gillnet fishery. The PBSAT meshes were much weaker than PA6 already at the start of the experiment. There were 58% (95% Confidence Interval (CI): 38%–79%) less cod and 32% (CI: 17%–49%) less plaice after 4 months in the PBSAT gillnets. We did not observe significant differences between the interactive effect of gear operation and degradation, and degradation only, and in capture modes. We conclude that reduction in catch efficiency results from a combination of weaker mechanical properties worsened by degradation during, e.g., transport and storage, and a species given shape and swimming ability.

Bycatch and discard rates assessment of the gillnet fishery in the southern Brazil

The fisheries discard patterns are an important element to be considered in stock assessment, fisheries management, and conservation measures. Using data collected by scientific observers on board, we analysed the bycatch and discard rates of the pelagic and bottom gillnet fishing in southern Brazil. Bottom gillnet was responsible for discarding from 77 taxa; 35 % of the discarded biomass was composed of teleost fishes, 33 % of elasmobranchs, and 22 % of other taxa. Pelagic gillnet, targeting the Pomatomus saltatrix, resulted in the discard of 13 species and taxa not identified at the species level, such as Cnidaria and Malacostraca. The most discarded fish species was Brevoortia pectinata, constituting 94.8 % of the total discarded biomass. The discard rate of bottom gillnets targeting Micropogonias furnieri was higher (20 %) than that of those targeting the demersal species (6 %), such as Cynoscion guatucupa (formely Cynoscion striatus) and Umbrina canosai. The high biomass proportion and numerous discarded species emphasize the need for effective management through temporal and spatial fishing restrictions. Our findings suggest potential measures, including the closure of fishing areas in coastal waters.

Seabird bycatch in European waters

AbstractThe incidental capture (bycatch) of seabirds is a global conservation issue and a top threat to European species that demands urgent conservation and management action. Here, we present the first European review of seabird bycatch data, considering all fishing gears and data collection methods available in the region. We calculate seabird bycatch numbers per species, family, country and European marine region and assess the reliability of the data available. The cumulative bycatch estimate extracted from this review suggests that about 195,000 seabirds (ranging from around 130,000 to 380,000) are bycaught in European waters annually. The most affected seabird species is the Common Guillemot Uria aalge with over 31,000 birds killed per year. The marine region with the highest bycatch estimate is the Northeast Atlantic (over 115,000 seabirds year−1). Gillnet fisheries are responsible for the highest bycatch levels, with over 95,000 seabirds year−1, followed by longline fisheries. The families most affected by bycatch are Anatidae and Alcidae. These numbers are likely an underestimation since we were unable to find bycatch estimates, or to extrapolate estimates from available bycatch data for 12 (out of 34) European coastal states. Our assessment also identified significant data gaps in key areas such as Gran Sol (in the north‐east Atlantic), the central and Eastern Mediterranean and the Black Sea. Combining systematic data collection with immediate implementation of mitigation measures will be crucial to fill in knowledge gaps, reduce current mortality levels and meet international conservation commitments such as those of the European Union and the Convention on Migratory Species.

Practices and informal institutions governing artisanal gillnet fisheries in Western Ghana

Practices and informal institutions governing artisanal gillnet fisheries in Western Ghana

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.

Use of floated gillnet to reduce flatfish bycatch in a mixed-species gillnet fishery

Bycatch of threatened flatfish species is a major concern in mixed-species bottom-set gillnet fisheries in the Yellow Sea, China. Therefore, we tested floated gillnets against standard bottom-set gillnets to reduce bycatch by exploiting difference in species-specific fish behavior. Our results demonstrated that the floated gillnet design can greatly improve species and size selectivity. Floated gillnets significantly reduced the catch efficiency of marbled flounder (Pseudopleuronectes yokohamae) and stone flounder (Platichthys bicoloratus) by 83% and 86%, respectively, compared to standard bottom-set gillnets. Additionally, catches of undersized target species, fat greenling (Hexagrammos otakii), black rockfish (Sebastes schlegelii), and Asian paddle crab (Charybdis japonica), were significantly reduced by 52%, 79%, and 78%, respectively. However, this reduced the catches of legal-sized fat greenling, black rockfish and Asian paddle crab by 21%–26%. The catch efficiency of target species black scraper (Thamnaconus modestus) was significantly improved. Furthermore, significant variations in species composition between floated and standard gillnets were observed. The findings of this study can provide a more sustainable management of the mixed-species gillnet fisheries.

An assessment of post-capture condition and survival of Rajidae caught in fixed nets

Skates (Rajiformes: Rajidae) are commercially and ecologically-important elasmobranchs captured in targeted and mixed fisheries globally. Many species are of conservation concern and are subject to high levels of discarding. They are currently exempt from the European Union’s landing obligation due to evidence of high discard survival, although the bulk of this evidence comes from mobile gear fisheries. Improved evidence on survival after discarding from fixed gears is necessary. We assessed immediate survival rates and post-capture condition (injury, reflex impairment, and vitality) of four skate species commonly discarded from seasonal Dover sole, Solea solea and plaice, Pleuronectes platessa gillnet fisheries in the Irish Sea and Bristol Channel, UK. We also modelled the relationship between a suite of environmental and operational fishing variables to the condition of thornback skate, Raja clavata. This is the dominant skate captured as bycatch and is classified as near threatened by the IUCN due to declining populations trends. At-vessel survival on gear hauling was high for all species (R. clavata = 99.43%, R. montagui = 100%, R. microocellata = 100%, R. brachyura = 100%). Post-capture condition of R. clavata was size-dependent, with larger skates demonstrating higher levels of injury and reflex impairment and poorer vitality. Higher vitality in smaller individuals and strong negative effects of aerial exposure on skate vitality highlight the likely benefits of prioritising discarding of unwanted catch upon gear hauling. This may maximise survival and future reproductive potential and facilitate population recovery.

Test of light emitting diodes (LED) as a possible bycatch mitigation measure in a gillnet fishery

Substantial seabird bycatch occurs in North Atlantic lumpfish fisheries, potentially impacting seabird populations and raising concerns about the fisheries’ sustainability. Despite increased efforts to address this issue, existing approaches have yielded inconclusive and mixed results. In this study, we explored an alternative approach by comparing bycatch levels between conventional gillnets and LED-illuminated gillnets during the 2023 Greenland lumpfish fishing season. Results indicated that LED-illuminated nets showed almost no difference in bycatch compared to no-LED nets for the three main bird species (common eider, king eider, and long-tailed duck), with no statistically significant differences observed. Additionally, the LED-illuminated nets exhibited a slightly lower, but insignificant, catch rate of the targeted female lumpfish. In conclusion, our findings suggest that using LED as a bird deterrent in this fishery is not an optimal solution to bycatch mitigation.

Unraveling the environmental influence on inter-annual fishery yield in a small-scale gillnet fishery under Rio de la Plata influence, South America

The study focuses on understanding the influence of the environment on the relative yields of a gill net fishery operating in San Antonio Cape, Argentina. The research was conducted during five austral warm periods covering spring to summer from 2008 to 2013, aiming to assess the potential impacts of climate change on small-scale fisheries. The hierarchical species composition of 563 fishery landings encompasses 41 registered species. Among these species, Cynoscion guatucupa, Micropogonias furnieri, Mustelus schmitti, and Parona signata were found to be dominant. The species composition showed a similarity ranging from 36.22% to 57.59%, with significant differences observed among most of the warm seasons. The Multivariate Regression Tree analysis showed that the temperature was the variable with the greatest influence on the composition of the landings. During warmer conditions (>20.4 °C), the location of the draft became a significant factor, while variation in turbidity played a role in the northern zone. In colder warm conditions (<20.4 °C), the composition differed based on the volumes of Río de la Plata discharge. Specifically, the freshwater outflow was further differentiated by depth (∼9 m) at lower volumes (<20 649 m3/s) and by turbidity at intermediate values (20 649–22 908 m3/s). The vulnerability of the fishery was explained by a combination of specific ranges of various stressors. These stressors led to significant variations in the composition of fishery landings, ultimately affecting the economic income of the fishermen. By understanding the relationship between environmental factors and fishery yields, this study provides valuable insights into anticipating the potential impacts of climate change on small-scale, vulnerable fisheries.

Evaluating the Sustainability of an Eastern Mediterranean Gillnet Fishery Based on the Catches of Undersized Individuals and the Reproductive Period of Targeted Species

The catch composition of a coastal gillnet fishery in the Eastern Mediterranean Sea was analyzed through a two-year experimental fishing survey. Seven fish species occurred regularly in the hauls. Surmullet, Mullus surmuletus, which is the most valuable demersal fish in Greek waters and the intended target of the gillnets in small-scale fisheries, was the most abundant and systematically caught species. Almost all surmullets were larger than their minimum conservation reference size. However, three commercially exploited species (Diplodus annularis, Pagellus acarne, and P. erythrinus) were caught systematically as undersized individuals. In addition, these three species were caught mostly as immature individuals. Moreover, the operational season of the surveyed métier overlapped completely with the reproductive period of five commercially exploited species (D. annularis, M. barbatus, M. surmuletus, Sphyraena sphyraena, and Trachurus trachurus). Improvements and the establishment of additional technical measurements should be considered for the small-scale gillnet fisheries in the studied area to attenuate their detrimental effects and achieve a better compromise between sustainable exploitation of the local multi-species fish resources and the need for an economically sustainable practice.

Investigation of Spatiotemporal Patterns of Harbour Porpoise (Phocoena phocoena) Strandings in Swedish Waters for Improved Monitoring and Management

Harbour porpoises (Phocoena phocoena) are the only cetacean residents found year-round in Swedish waters and they are exposed to numerous natural and anthropogenic threats. Since the in situ monitoring of cetaceans can be difficult, invasive and often expensive, investigation of stranding patterns and examination of stranded animals can be used as a cost-effective source of data to study these elusive animals. The aim of this study was to investigate the spatiotemporal patterns of harbour porpoise stranding reports and the possible underlying causes in Swedish waters over a ten-year period (2014–2023). Additionally, the Swedish stranding network plays a key role in the collection of stranded carcasses for health and disease surveillance, and geographic coverage of the network also was analysed. When making spatial comparisons, the ten-year period was divided into two five-year blocks. Data on 854 stranded harbour porpoises were analysed from the coasts of the Skagerrak, Kattegat, and Baltic Seas. Both significant spatial and temporal patterns could be identified. Strandings peaked in July through September and hotspots occurred along most of the Swedish west coast, with the most frequent hotspots located around Öresund and especially the area around the Kullen peninsula. The spatial patterns of strandings found in this study are consistent with data on porpoise abundance, prey abundance, and gillnet fisheries’ efforts. The latter is known to be one of the primary causes of porpoise mortality. Furthermore, the coverage of the Swedish stranding network increased between the two periods, likely reflecting an increased awareness of the carcass-based surveillance program, and gaps requiring network expansion efforts were identified. These results also provide baseline data to enable the continued monitoring of stranding trends, as changes may indicate changes in population distribution, size or mortality rates.

Incidental take of Giant Sea Bass in the gill net fishery

The Giant Sea Bass is an apex predator of southern California kelp forests that historically supported large commercial and recreational fisheries but whose population was drastically reduced due to overfishing. Although regulations have been in place to protect Giant Sea Bass for over 30 years, there is concern for continued allowance of their incidental take in commercial gill net fisheries. Through landing records, logbooks, and federal observer data, we examined the commercial take of Giant Sea Bass to determine the: magnitude, location, age, and disposition of catch; whether this take was incidental or targeted; and its impact on the current population. We found that Giant Sea Bass landings declined since gill netting was banned in state waters in 1994 and their overall landings decreased by 75% since becoming protected in 1981. Over time, a greater proportion of older fish was seen in the landings with an increase in mature individuals over 13 years of age since 2000. We found no evidence that commercial fishers are targeting Giant Sea Bass. On average, Giant Sea Bass were landed on 4.2% of gill net trips from 1994 to 2021. Additionally, over the past decade the majority of Giant Sea Bass landings came from only 15 permit holders, but this made up only ~2% of their total landings in pounds as these are the most active permittees, bringing in the highest landings of White Seabass or California Halibut. Assessing gill net observer data, we found the mortality of released Giant Sea Bass is high at 72%, although despite this level of mortality, multiple fishery independent sources indicate Giant Sea Bass populations are increasing, suggesting that current protection measures may be effective. Despite these findings, it is challenging to interpret the full impact of the commercial fishery on Giant Sea Bass as an accurate measure of total discards is unknown without consistent observer coverage.

Critical areas for sea turtles in Northeast Brazil: a participatory approach for a data-poor context.

Fishing is one of the main threats to sea turtles due to the risk of entanglement in lost nets, vessel collision and mortality due to incidental catches. In Brazil, most of the studies regarding fishing interactions with sea turtles are focused on pelagic longline fisheries in the South and Southeast regions. However, their main reproductive areas in Southwest Atlantic RMU occur in Northeast Brazil, which overlaps small-scale coastal gillnet fisheries. Here, we aimed to use ethnobiology and participatory approaches as simple and cost-effective methods to identify areas for sea turtle conservation where impacts from small-scale fisheries are most likely. Expert captains were trained to recording sea turtle sightings during navigations from the landing port to the fishing grounds, informing their folk nomenclatures. By interpolation of environmental data (chlorophyll and bathymetry) and ecological data, we predicted habitats of 3,459.96 km² for Caretta caretta, Chelonia mydas, and Eretmochelys imbricata and fishing zones of 1,087 km² for management in 20 m and 50 m depths. Our results contributes to ongoing discussions of bycatch mitigation for sea turtle species and identification of habitats. We highlights the importance of considering particularities of overlapped areas in marine spatial planning and co-management arrangements.

Livelihood status of the fishing community of Mahakanadarawa reservoir in Anuradhapura, Sri Lanka

Mahakanadarawa reservoir located in Anuradhapura is one of the major, perennial reservoirs in Sri Lanka, extending over 1,457 ha. It provides livelihood opportunities through fisheries, irrigation for agriculture, and fostering economic growth in the surrounding region. Yet the fisheries-based livelihoods of the Mahakanadarawa reservoir have not been fully assessed. Hence, this study investigated the fishery and fisher profile of the reservoir. Primary data were collected from eight landing sites. A pre-tested questionnaire was used to interview 72 fishermen using a mixed method. Main variables including social, human, physical, natural, and financial capitals, fishery profile, fish and fisheries-based post-harvesting techniques, market chain analysis, and fisheries management practices, catch per unit effort for the gillnet fishery were calculated. Secondary data including stocking fish fingerlings and species-wise harvesting data for the past 10 years were collected from the district aquaculture extension office (Anuradhapura) of the NAQDA. A secondary study on the length-weight relationship of Oreochromis niloticus along with its condition factor value was performed. It was revealed that O. niloticus had negative allometric fish growth (b = 2.6667) with a condition factor value of 1.9092. The female fish had a higher mean total length compared to the male fish. In the fishing community, 95.83% of males and 4.17% of females were engaged in fishery activities. Mainly gill nets and non-mechanized fiberglass canoes were used for fishing. The average CPUE was 0.000339 kgm-2day-1. A strong positive (r =0.841) correlation was reported for fish stocking and fish production. Fish smoking and sun drying were the common fish processing techniques. The fish supply chain targeted the regional market. A cooperative level co-management system was identified among the reservoir fishing community. The current study discloses the livelihood capitals of the fishing community along with the existing supply chain which can be utilized for the sustainable management strategy of the Mahakanadarawa reservoir in the long run.

Struggles beneath the waves: Unveiling the incidental capture of sea turtles by artisanal fisheries in Angolan waters

The modernization and intensification of fishing practices have raised the threat to sea turtles, with escalating unintentional captures endangering turtles of all life stages. In this study, we examined the impact of artisanal fishing on sea turtles in two coastal regions of Angola, southern Africa. Between August 2015 - March 2016, through participatory monitoring (n = 8) and interviews with fishers (n= 41), we documented unintentional turtle captures occurring year-round during fishing activities. A total of 405 turtles, 403 olive ridley (Lepidochelys olivacea) and 2 green (Chelonia mydas), were caught in gillnets by fishers engaged in participatory monitoring. Net characteristics, including length and proximity to the shoreline, as reported by the interviewed fishers, correlated with those used by the monitoring group. We estimated approximately 1219 turtles captured by the 41 interviewed fishers over the 8-month study period. Most captures occurred within 100 m from the shoreline. The likelihood of accidental turtle captures increased significantly with proximity to the beach, larger mesh sizes, and greater net lengths and number of hauls. We calculated a survival rate of 41%, and a mortality rate of 59% for captured turtles. Number of turtles caught as bycatch varied throughout the year, with the highest Catch Per Unit Effort (CPUE) reported for Oct. – Dec. Since this period aligns with the nesting season, most captured turtles are likely to be females. The high mortality rate reported in this study emphasizes the immediate threat posed by accidental captures to sea turtle populations, underscoring the critical importance of conservation efforts, particularly in artisanal gillnet fisheries.

Effect of mesh size in monofilament and multifilament gillnets on catch efficiency in the Black Sea whiting (Merlangius merlangus) fishery

Gillnets are widely used fishing gear for targeting various fish species. Gillnet fisheries use different gear configurations such as different mesh sizes and number of netting twines (mono- or multifilament) depending on the species targeted. Gillnet fishery targeting whiting (Merlangius merlangus) is one of the economically important year-round fisheries in the Black Sea. However, large bycatch of whiting below minimum landing size (MLS) has led to a stock decrease, questioning sustainability of this fishery. This study evaluated how netting twine construction and mesh size can affect the capture probability of whiting. The results showed that changes in gillnet mesh size significantly affect the capture probability with increased mesh size reducing the average capture probability of undersized whiting from around 60%–5%. The results showed no significant differences in the average catch efficiency between monofilament and multifilament gillnets of the same mesh sizes; however, monofilament gillnets showed an increased catch efficiency for some length classes of whiting > MLS. Based on these results, use of 32 or 34 mm mesh size monofilament gillnets is advisable for sustainable whiting fisheries management in the Black Sea.

Impact of fishery observer protocol on estimated bycatch rates of marine mammals

Abstract Managing fishing operations’ threat to marine mammal populations hinges on accurate bycatch estimates, often derived from fishery observer or monitoring programmes. Much global marine mammal bycatch occurs in gillnets, and observer protocols that do not include watching the haulback of gillnets may miss animals that drop out of the net. We investigated whether trips using a fish-focused observer protocol (no requirement to watch the haulback) in US northwestern Atlantic gillnet fisheries from 1994 to 2019 had different observed bycatch rates from trips under a mammal-focused observer protocol (watching the haulbacks) for grey seals (Halichoerus grypus atlantica), harbour seals (Phoca vitulina vitulina), and harbour porpoise (Phocoena phocoena phocoena). We found that observer protocol was likely to affect observed drop-out and bycatch rates. Under the fish-focused protocol, the ratio of animals removed from the net to those that fell from the net was generally higher than under the mammal-focused protocol, suggesting fish-focused observers missed bycatch that fell. Bycatch rates of animals removed from the net by fishers differed significantly between observer protocols for seals, but not for harbour porpoise, perhaps because of differences in entanglement and manner of decomposition. We estimate bycatch was underreported by 3–25% because of unobserved drop-outs on fish-focused observer protocols.