Lophius Piscatorius in Northern European Waters
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INTERNATIONAL COUNCIL FOR CM 2001/J:27 THE EXPLORATION OF THE SEA The Life History, Dynamics and Exploitation of Living Marine Resources: Advances in Knowledge and Methodology WHERE ARE THE MATURE ANGLERFISH? – THE POPULATION BIOLOGY OF LOPHIUS PISCATORIUS IN NORTHERN EUROPEAN WATERS. Chevonne Laurenson, I.G. Priede, L.W. Bullough and I.R. Napier ABSTRACT The general relationship between length frequency and water depth was investigated for anglerfish (monkfish) Lophius piscatorius Linnaeus 1758 in waters around the Shetland Isles, Scotland. Length and age data were obtained during commercial trawling trips between 1998 and 2000 and correlated with depth. A trend of increasing average length (and age) with increasing depth was found, in accordance with Heincke’s Law. Within the length frequency distributions modes thought to represent year classes correlate with mean lengths-at-age derived from otolith readings. Analysis of maturity stages suggests that the average length at first maturity is 59cm for males and 96cm for females. Such fish represent only 12% and 2% respectively of the commercial catch. The high proportion of immature anglerfish, particularly females, within the fishery is a cause for concern. A Harden-Jones style triangle is presented with immature fish predominating in the inshore and shelf waters and a general movement into deeper waters with increasing size and maturity. It appears however that even in deep water mature anglerfish are relatively scarce. It is speculated that the triangle is completed with a planktonic drift of eggs and larvae towards shallower waters before settlement. C. Laurenson, L.W. Bullough, I.R. Napier: North Atlantic Fisheries College, Port Arthur, Scalloway, Shetland, U.K. [tel: +44 (0)1595 880328, fax: +44 (0)1595 880549, e-mail: [email protected], [email protected], [email protected]]. I.G. Priede, University of Aberdeen, OceanLab Culterty, Newburgh, Aberdeen, U.K. [tel: +44 (0)1358 789631, fax: +44 (0)7775 866971, email: [email protected]]. Introduction In the past, anglerfish were taken as a by-catch (and discarded) during bottom trawl groundfish fisheries (1999; Afonso-Dias, 1997; Fahy and Gleeson, 1992). However, a targeted fishery for Lophiid species has developed in ICES Areas IV and VI over the last 10-15 years. The majority of the UK catch of anglerfish is taken in Scottish waters (Afonso-Dias, 1997). The fishing grounds to the north and west of the Shetland Isles (covered during this study) became important within the targeted anglerfish fishery during the 1990’s. Statistics show that in Scotland the landings peaked at 26.1 thousand tonnes, worth £44.8 million at first sale during 1996 but this fell steadily to 12.1 thousand tonnes, worth £28.4 million during 1999 (2000b). To put their value and landings into perspective, during 1996 cod Gadus morhua and haddock Melanogrammus aeglefinus landings were worth £38.9m (42,300 tonnes) and £49.8m (82,300 tonnes) respectively. Anglerfish are species whose maximum value is fresh rather than processed and for which continental Europe has provided the main markets (Fahy and Gleeson, 1992). During the present study, which covered grounds around the Shetland Isles (Figure 1), with additional data from grounds to the west of the Hebrides it was found that Lophius piscatorius Linneaus, 1758, the white-bellied anglerfish was the predominant species within the fishery. This species is sold in Scotland under the trade name of (and is commonly known as) ‘monks’ or ‘monkfish’. L. piscatorius represented 99% of anglerfish catches. The remaining 1% was L. budegassa Spinola, 1807, the black- bellied anglerfish, or monkfish whose distribution is concentrated more in the Mediterranean. Similarly Afonso-Dias, (1997) found that L. budegassa represented not more than 3% of catches to the northwest of Scotland. The species are not separated during selling or processing. Although the fishery for Lophiid species has become of economic importance to the trawling fleet in recent years, much basic information on the biology, ecology and abundance is still lacking. It is considered that for the North Sea and West of Scotland fishing mortality and recruitment are not well estimated (2000a). The purpose of this study was to gather, first hand, information relating to the anglerfish catches on the grounds around the Shetland Isles. From this, their length frequency and age have been investigated in relation to water depth and biological features of the species and its fishery, and are discussed in relation to the possible implications for its future management. Materials & Methods Between May 1998 and June 2000 data collection trips were conducted onboard commercial whitefish trawlers belonging to the Shetland fleet. A total of 286 hauls were sampled covering 1,621 hours trawling time. All 11,257 anglerfish caught were measured to the nearest centimetre below (total length). For the purpose of age determination, sagittal otoliths were collected from a representative sample of the catch. Additional otolith samples were obtained from market sampling of L. piscatorius landed in Shetland. A total of 2428 pairs of otoliths were collected, stored in and read under a 1:1 glycerine: water solution following the methodology of Tsimenidis and Ondrias, (1980). Otoliths were used as the ageing structure partly due to their ease of removal, storage and preparation but also because samples were taken from commercial catches and their removal did not affect the market value. As all landings are of gutted fish, determination of sex and maturity stage was done de visu using the scheme of Afonso-Dias and Hislop, (1996). Maturity stage was assigned to 2137 males and 2207 females. For each tow the shooting and hauling times and positions were recorded, as was the tow track. On most of the sampling trips water temperature and depth were recorded every 2 minutes using a Vemco minilogger attached to the net otherwise depths were recorded from the wheelhouse echosounder. During the summer of 1999 additional anglerfish data was collected during sandeel (Ammodytes marinus) surveys in inshore waters around Shetland. Data collected by one of the authors’ (L.B.) from grounds to the West of the Hebrides (depth range 150 - 850m) has also been included in the analyses. Kolmogornov-Smirnov 2-sample tests (Sokal, 2000) were carried out to compare length frequency distributions over the depth ranges. Results Geographical area Figure 1 shows both the area covered during observation trips in waters around the Shetland Isles and the area to the west of the Hebrides from where additional data was obtained. Age determination The mean lengths-at-age determined from otolith readings are shown in Table 1 and the resulting growth curve in Figure 2. As can be seen, the growth curve agrees closely with those from previous studies, confirming the accuracy of age determination in this study. Table 1. Mean lengths-at-age for L. piscatorius determined from otolith readings. Age Mean Age Mean (years) length (years) length (cm) (cm) 0 17.9 7 69.2 1 28.9 8 74.0 2 36.4 9 79.7 3 43.3 10 86.3 4 50.0 11 93.0 5 56.6 12+ 109.2 6 62.6 Length frequency in relation to fishing depth Figure 3 represents percent length frequency distributions of L. piscatorius caught in different depth ranges. The mean lengths at ages as determined during the present study are superimposed over the percent length frequency distributions. A progressive shift towards predominantly larger and older fish with increasing depth is apparent. In Table 2 summary data from the length frequency distributions is presented for each depth range, mean age at each depth is also shown. The Kolmogornov-Smirnov tests showed significant differences between the length frequency distributions in each successive depth range (P> 99% in each case). Mean length data from each depth range within the Shetland area (Table 2) are plotted against the mid-point of each depth range and shown in Figure 4. A strong linear relationship exists (y = 5.32x – 116.9, r = 0.995). Table 2. Summary data from the length frequency distributions at each depth range. Depth Length Mean Mean Age N Range (m) Range (cm) Length (cm) (years) 1 <50m 4 – 54 27 1.3 132 1 50-99m 16 – 54 35 2.2 626 1 100-149m 16 – 103 47 3.7 9331 1 150-199m 13 – 124 54 4.6 1213 2 150-300m 28 – 71 52 4.4 205 2 600-850m 22 - 125 64 6.1 473 1 Shetland waters, 2 West of Hebrides. Maturity The sex ratio for each size-class sampled are shown in Figure 5. For anglerfish below about 58 cm the sex ratio was approximately 1:1, but above this size the proportion of females increases with length. Fish above 91 cm in length were exclusively female. Figure 6 shows the maturity ogives for each sex by length (based on macroscopic observations). The L50 for female L. piscatorius is 98cm compared to 58cm for males. From the length frequency distributions at each depth range (Figure 3) and the sex ratio and maturity ogives (Figures 5 and 6) the percentage of mature L. piscatorius (capable of spawning: Stages III-V) has been estimated for each of the depth ranges sampled. This indicates a general trend of increasing percentages of mature fish within the population with increasing depth. Higher proportions of males than females were reproductively active at all depth ranges sampled. Table 2. Estimated sex ratio of L. piscatorius and percentages of mature males and females in each depth range sampled (based on length frequency data). Depth m Male : Female % Male spawners % Female spawners within total population within total population 1 <50m 1 : 1.00 0.9 0.2 1 50-99m 1 : 1.01 4.6 0.8 1 100-149m 1 : 1.07 11.2 2.2 1 150-199m 1 : 1.10 16.9 3.3 2 150-300m 1 : 1.01 15.8 1.7 2 600-850m 1 : 1.27 25.7 6.5 1 Shetland waters, 2 West of Hebrides.