Implications of reducing Norway lobster (Nephrops norvegicus) minimum size (MLS/MCRS) in the Skagerrak and Kattegat area (IIIa).

Transcription

1 Institutionen för akvatiska resurser Havsfiskelaboratoriet Daniel Valentinsson, Joakim Hjelm and Katja Ringdahl Implications of reducing Norway lobster (Nephrops norvegicus) minimum size (MLS/MCRS) in the Skagerrak and Kattegat area (IIIa). Background (management) The MLS (Minimum Landing Size) for Nephrops is currently 40 mm carapace length (CL) in the Skagerrak/Kattegat (Area IIIa) compared to 25 mm in the North Sea. The relatively large MLS results in high levels of discards of small, undersized Nephrops and it has been discussed, for many years, a need to increase the size selectivity in Nephrops trawls in order to decrease the discard rate (ICES 2004). A traditional criterion for setting MLS has been to ensure that the majority of females have had the possibility to spawn at least one time before being caught. In the revised CFP (Regulation EU 1380/2013), MLS has been replaced by a MCRS (Minimum Conservation Reference Size). The MCRS defines how large a caught individual species need to be in order to be sold to human consumption. Individuals smaller than MCRS will also have to be landed and subtracted from quotas but can only be used for purposes other than human consumption. The likely price difference between catch above and below MCRS may thus create a strong incentive to avoid catching small specimens by for example increased selectivity. Regarding MCRS, the basic regulation states that: 'minimum conservation reference size' means the size of a living marine aquatic species taking into account maturity, as established by Union law, below which restrictions or incentives apply that aim to avoid capture through fishing activity; such size replaces, where relevant, the minimum landing size; (Art 4:17) Minimum conservation reference sizes may be established with the aim of ensuring the protection of juveniles of marine organisms (Art 15:10) The procedure to establish or change MCRS is furthermore detailed in article 18a of the "Omnibus Regulation" (PE-CONS 11/ /0436 (COD) meeting version exp meeting ), which repeats that the purpose is to ensure the protection of juveniles. As a consequence of the revised CFP, a proposal to harmonize minimum sizes between the North Sea and area IIIa has been put forward within the Scheveningen group. This would mean a reduction from 40 mm to 25 mm in carapace length. The possible implications of such a reduction is elaborated upon in the following sections. 1

2 Background (biology) Discard mortality in Nephrops is highly variable and depends on a many of factors such as the degree of damage incurred during capture and post-capture treatment, air temperature and the of predation by seabirds, fish and other marine predators during their return to the sea-bed (Chapman 1981, Campos et al. 2004, Harris and Ulmestrand 2004). In general though, discard mortality is much higher for trawl caught than for creel caught Nephrops. ICES currently use 75% trawl discard mortality based on Wileman et al. (1999) for stock assessment purposes. The discard mortality figure used in stock assessment has large implications on the exploitation rate, particularly as the landing obligation most likely will result in quota uplifts due to that previous discards will be added to landings to form a catch quota. Sexual maturity in Nephrops is known to differ between males and females and between different functional units. An ICES workshop examined maturity for a wide range of European Nephrops populations and found that average length at maturity (L m 50) for female Nephrops are generally higher in the Skagerrak and Kattegat than in the adjacent North Sea area (ICES 2006; Figure 1 and 2). They further reported that there are no systematic differences between other stocks in the North Sea and western waters (FUs 5 to 24). Figure 1. Maturity for female Nephrops in the Skagerrak. Graph from ICES (2006). Figure 2. Overview of sizes at maturity (L m 50) in different Nephrops functional units (FU). FU 3-4: Skagerrak and Kattegatt; FU 5-24: North Sea and western waters. Graph from ICES (2006). 2

3 In Table 1, examples of average proportion of mature females for four different sizes are shown. Table 1. Maturity of female Nephrops in IIIa. Data extracted from ICES Another potential criterion to base MCRS upon is L opt. Froese et al. (2008) showed that fishing has much less impact on stocks if fish are caught after they have reached the size (L opt ), i.e. the size where growth rate and cohort biomass are maximized. However, in comparison to size at maturity, the use of L opt is not clearly as established in the CFP basic regulation when establishing MCRS. L opt is calculated according to the equation:!!"# =!!"# (!! ) where L inf and K are parameters of the von Bertalanffys Growth Function and M is the instantaneous natural mortality. L inf and K are 73 and 0.14 for males and 65 and 0.10 for females and M is 0.2 and 0.3 for females and males respectively (Ulmestrand and Eggert 2001). L opt for Nephrops in IIIa is then estimated to 42.6 mm for males and 39.0 mm for females, which is around current MLS. The behaviour of berried Nephrops females is to hide in the burrows for several months without emerging, implying that there is a buffer of berried females that are not available to trawls. This means that females are normally underrepresented in trawl catches and that the behaviour may act as a buffer from high fishing intensity. The assessment of the Nephrops stock in the Skagerrak and Kattegat is based on an internationally coordinated underwater TV survey where numbers of Nephrops burrows per area are estimated for each station and scaled up to the total Nephrops habitat in the area. The survey from 2013 showed a harvest rate (number of dead removals/total abundance) of 5.8%, meaning that around 94 % of the Nephrops stock in IIIa was not exploited during Reduced MLS - analysis and results Figure 2 shows the number and weight of landings and discards given constant catch for four different MLS (40, 35, 30 and 25 mm CL, corresponding to 134, 118, 103 and 87 mm total length; TL), and how the discard proportion and mortality will change in the different MLS-scenarios. 3

4 Figure 3. Size distributions (average for ) of landings and discards in a constant catch scenario for different MLS. Tabled below each figure is numbers (in million) and weight (ton) for the different scenarios. 4

5 Figure 3 shows that using the average Nephrops length frequency distribution from observed catches in IIIa during (ICES data) and current MLS of 40 mm CL, the landings were 3922 tonnes and 3749 tonnes (49%) of the catch was discarded as undersized. If a MLS of 25 mm was used (currently legislated MLS in the North Sea), the landings would be 7582 tonnes and only 89 tonnes (1.2%) would be discarded back to sea. Note that as the MLS is reduced the number of dead removals is increased (from 6734 to 7649 tonnes; Fig. 3). This is due to that the discarded Nephrops that survives in a high-mls scenario will be landed in a low- MLS scenario, and implies that the catch quota needs to be reduced in order not to increase fishing mortality. The corresponding figures for 30 and 35 mm CL can be found in figure 3. Another consequence to consider of a reduced MLS is that the Swedish Nephrops quota allocation may need to be revised (currently 25% is allocated to creels, 50% to grid trawls and 25% to conventional trawls). A reduced MLS/MCRS will result in a higher relative quota uptake for fisheries with a large proportion of small individuals (i.e. trawl fisheries) than in the creel fishery where the proportion of Nephrops smaller than the current MLS is considerably lower in their catches (typically 10% or less). Other options to compensate for this imbalance between fishers may be needed nationally. Conclusions and recommendations The current MLS (future MCRS) of 40 mm CL allows most female Nephrops to spawn at least once and in in accordance with L opt. However, the mis-match between MLS and selectivity results in high discard rates (typically 50% as annual average). The proposed reduction in MCRS to 25 mm would almost eradicate discards but is most likely not in accordance with the basic regulation for Nephrops in the Skagerrak/Kattegat as only app. 10% of females in the area are mature at this size. There is thus no biological basis for harmonizing MCRS across the wider North Sea region. Furthermore, the incentive to avoid capture of juveniles, as stated in the basic regulation, is lost. Another risk with such a drastic size reduction is implementation failure due to that the catch (and landing) opportunities would increase significantly (see Figure 3). A likely implementation failure is that the increased catch quotas will incentivize a continued focus on larger Nephrops (and continued discarding of small), which can result in a higher exploitation rate than intended. A more modest reduction in MCRS can most likely meet the basic regulation objectives to take account of maturity and to create incentives to avoid capture of juveniles. At the same time discards will be reduced. If MCRS is set at L m 75, this would correspond to a MCRS of app mm CL (110 mm total length). Correspondingly, if MCRS would be set at L m 50, this would correspond to a MCRS of app mm CL (104 mm total length). These scenarios would reduce average annual discards from 50% to app. 10 and 6% respectively, but at the same time keep some incentives to reduce the capture of juveniles. These examples are somewhat arbitrary (and can be rounded off) but can hopefully be used as guidance as the setting of MCRS in relation to the basic regulation is not a scientific question. The high discard mortality currently used in stock assessment and the estimated low harvest rate for Nephrops in IIIa suggest that such a decrease in MLS/MCRS will most likely not change the state of the Nephrops stock if exploitation rate remains at current level. 5

6 References Campos, A., Fonseca, P., Mendes B., Pilar-Fonseca, T., Castro, M., Leocadio, A., Survival of Norway lobster (Nephrops norvegicus) escaping from trawl cod-ends. FCT Contract nº PDCT/MAR/59366/2004, Final Report. Froese R., Stern-Pirlot A., Winker H. and D. Gascuel, Size matters: How single-species management can contribute to ecosystem-based fisheries management. Fisheries Research 92, Harris, R. R. and Ulmestrand, M Discarding Norway lobster (Nephrops norvegicus L. ) through low salinity layers - mortality and damage seen in simulation experiments. ICES Journal of Marine Science. 61: ICES The Nephrops fisheries of the Northeast Atlantic and Mediterranean A review and assessment of fishing gear design. ICES Cooperative Research Report. No pp. ICES Report of the Workshop on Nephrops Stocks (WKNEPH), January 2006, Copenhagen, Denmark. ICES CM 2006/ACFM:12. Ulmestrand, M., and Eggert, H Growth of Norway lobster, Nephrops norvegicus (Linnaeus 1758), in the Skagerrak, estimated from tagging experiments and length frequency data. ICES Journal of Marine Science, 58: Wileman, D.A., G. I. Sangster, M. Breen, M. Ulmestrand, A. V. Soldal and R.R. Harris, Roundfish and Nephrops survival after escape from commercial fishing gear. EC Contract No: FAIR-CT Final Report p + appendix. 6