Set a minimum landing size for commercially fished species
Overall effectiveness category Awaiting assessment
Number of studies: 5
Background information and definitions
Fishing can impact marine fish through species removal or habitat damage from fishing gear contacting the seabed. Removal of small fish before they have matured and had an opportunity to reproduce limits the overall reproductive potential of the population and can cause severe depletions. The minimum landing size is the smallest fish measurement (usually total length) at which it is legal to retain a fish and is dependent on species (e.g. size at maturity) and sea region. Setting a minimum size at which fish can be landed may help ensure that fish grow to a size that gives them the opportunity to spawn at least once, thus helping to maintain the population. However, to ensure that discarding of fish below a given minimum size is minimized, the size-selectivity of the fishing gear may also need to be improved to better match the size limit.
For similar interventions, see ‘Protect Reproductive Individuals - Set a maximum landing size for commercially fished species’ and ‘Specify a size range of capture for commercially fished species’.
Supporting evidence from individual studies
A review in 2000 of broadbill swordfish Xiphias gladius fisheries worldwide (Ward et al. 2000) reported that after recommended minimum landing sizes and catch limits were introduced, strong recruitment was found for one stock, whilst four others could not be assessed. This result was not tested for statistical significance. After measures were introduced in 1994, recruitment of age-1 swordfish in the North Atlantic area was strong in 1997 and in 1998 (no data reported), while in the four other areas there were either wide fluctuations in recruitment or there was no reliable assessment of the stocks (see paper for details). In addition, compliance was generally poor and between 19–37% of swordfish landed from various countries fishing in the Atlantic in 1998 were under the recommended size (125 cm). In 1994, minimum size limits and total allowable catches for swordfish were recommended by the International Commission for the Conservation of Atlantic Tunas. However, the recommendations were enforced by some, but not all member states, and following the limits some vessels relocated to other regions and an increase in discarded swordfish was reported. Five swordfish fisheries were reviewed to develop guidelines for the assessment and management of developing swordfish fisheries.Study and other actions tested
A replicated, before-and-after study in 2001 of three estuaries in the Tasman Sea, Australia (Gray et al. 2004) found that the effect of an increase in the minimum landing size for dusky flathead Platycephalus fuscus varied between estuaries, and there was an increase in the amount of discarded (undersized) flathead in gillnets in one of three estuaries. Catch rate of undersized dusky flathead was similar in the period after the increase in minimum size compared to before in two of the three estuaries (after: 0.05–0.14 fish/100 m, before: 0.04–0.16 fish/100 m) and was higher after in the other (after: 0.46 fish/100 m, before: 0.04 fish/100 m). In addition, no difference was seen in catch rates of commercial-sized flathead across all estuaries (after: 2.6–4.6 fish/100 m net, before: 4.1–4.8 fish/100 m net). Minimum landing size of dusky flathead was increased from 33 cm to 36 cm on 1st July 2001. A total of 81 commercial gillnet catches targeting flathead were sampled by scientific observers before and after the change in minimum size: in February-June and July-November in two estuaries (Wallis Lake and Tuggerah Lake), and in May-June and July-August in another estuary (Lake Illawarra), all in New South Wales. All species caught were identified, counted and the total weight recorded.Study and other actions tested
Referenced paperGray C.A., Johnson D.D., Young D.J. & Broadhurst M.K. (2004) Discards from the commercial gillnet fishery for dusky flathead, Platycephalus fuscus, in New South Wales, Australia: spatial variability and initial effects of change in minimum legal length of target species. Fisheries Management and Ecology, 11, 323-333.
A before-and-after study in 2002–2003 in a heavily fished area of seabed in the Baltic Sea, Northern Europe (Suuronen et al. 2007) found that an increase in minimum landing size did not reduce the amount of Atlantic cod Gadus morhua discarded by the trawl fishery for this species. This result was not tested for statistical significance. In the year following the minimum landing size increase in 2003 the discard rate of cod was 0.23 (rate by fish numbers) and 0.14 (rate by fish weight). In the year before the increase the discard rate was 0.18 by number and 0.09 by weight. The minimum landing size of Baltic cod was increased from 35 cm to 38 cm in January 2003. However, in April 2003, high discarding led to the temporary closure of the trawl fishery in Baltic European Union waters. Measures to improve selectivity of fishing gear in line with the increased minimum landing size were implemented in September 2003 (see paper for details). The typical trawl configuration in use at the time of the intervention was a 130 mm diamond-mesh codend. Discard data was collected from the Swedish cod fishing fleet by on-board observers.Study and other actions tested
A replicated study in 2004–2005 of fishing grounds in the Ionian Sea, Greece (Stergiou et al. 2009) found that the minimum landing sizes set for 13 fish species were smaller than their estimated lengths of maturity and in commercial landings over 50% of fish were immature, indicating that minimum sizes did not prevent most fish from being caught before they had a chance to spawn even once. These results were not tested statistically. For 13 fish species, the minimum landing sizes (total length) in use were between 2–38 cm smaller than the estimated size at which 50% of fish of a given species become mature (see paper for species individual data). The average percentage of immature individuals landed (smaller than the 50% maturity size) was greater than half of the catch for all gear types (55–92%). In addition, the average percentage of fish landed with lengths smaller than the minimum size varied between gear types and the overall average (all species) ranged from 6% (longlines) to 43% (beach seines). Data were collected from the landings of 22 vessels fishing out of Zakynthos Island between July 2004–2005. Total length of all fish was measured from landings by five gear types: trawls (11 trips), purse seines (27 trips), beach seines (3 trips), trammel nets (111 trips) and longlines (34 trips).Study and other actions tested
A before-and-after study in 2000–2007 of an estuarine system in the Northwest Atlantic Ocean on the North Carolina coast, USA (Smith & Scharf 2010) reported that management measures including increasing the minimum size limit and mesh size of gill nets in a commercial southern flounder Paralichthys lethostigma fishery reduced the catches of the youngest, but not older fish at two of three sites, compared to before. Relative frequency of age one flounder in catches from two different areas decreased in the two years following management measures, compared to the previous five-year period (after: 24–27%, before: 45–55%). Catches of older (age two and three) fish increased (after: 20–50%, before: 5–45%). However, at a third site, flounder catches after management changes were made were predominantly age one, immature, fish (90%) indicating only a small proportion of the harvestable stock would have the opportunity to reproduce (no figures reported for the period before changes). In 2005, management changes included a 25-mm increase to 356-mm in flounder minimum size limit and increased minimum mesh size for large-mesh gillnets to 140-mm. Southern flounder age and length data were collected during the period after management changes (2005–2007) from 1,970 legal-sized flounder caught by commercial fishers and tagged and released in the New River. A total of 627 of these were recaptured, mainly by the commercial gillnet fishery. Catch-at-age patterns for other flounder commercial gillnet fisheries in the state were estimated for 2000–2006 by combining length-specific regional landings with annual age-length data.Study and other actions tested