The shark angler and conservation in the UK: A Blue Shark tagging study from Looe.
Ever since its inception in 1953, the Shark Angling Club of Great Britain, has been involved with the conservation of shark species and as Brigadier Caunter noted in his 1961 book, Shark Angling in Great Britain, early collaborative tagging existed between the club and government agencies, in that case the,( as then) Ministry of Agriculture, Fisheries and Food (MAFF).
Early tagging studies were basic, a tag with information attached to the shark and (hopefully) the tag returned upon recapture. Although tags have evolved since those early days and electronic tags, which can record with more precision the travels of fish across and within the oceans, have been developed, these methods are expensive and there is still a value of simple cheaper tagging studies and the information collected can be used to inform fisheries managers through insights into the movements and life histories of shark species.
The SACGB still has active involvement in tagging studies, through the excellent US-based National Oceanic and Atmospheric Administration’s (NOAA) Cooperative Shark Tagging program (CSTP) which has tagged over 295,000 sharks globally since its inception in 1962. NOAA actively encourage citizen science and the resulting programs produce both an excellent data set and a sense of involvement for anglers with fisheries management.
From 1998-2011, the SACGB partnered with the UK Shark Tagging Program to apply tags to, mainly Blue Sharks and although now defuncted, the information produced can still aid our understanding of the Blue Sharks and its epic migrations throughout the Atlantic Ocean.
Figure 1 Average weight (left) and length (right) of sharks tagged from 1998-2011 from Looe. Dashed lines indicate long term averages.
Historically immature female fish dominated the UK Blue Shark population and of the 1440 sharks tagged from the period 1998-2011, 1225 were females, 130 males and 85 were not identified, giving a ratio of female to male sharks of 9.42:1. Only 8.61% of female and 6.92% of male fish were mature upon capture. The smallest fish tagged was a female of 35.83 inches (11 lb), whist the largest was a female of 107.09 inches (169 lb). The mean length and weight varied throughout the study from a high of 75 lb in 2009 to a low of 45.76 lb in 2011 (Figure 1.). Overall, the mean size of male fish tagged was lower than that of females (Figure 2.) but the range of size classes present was higher for males (Figure 3.). This data allowed the calculation of several important fisheries parameters for the Blue Shark, such as the synoptic length (average maximum) of 107.43 inches and the theoretical longevity of 23.10 years
Figure 2 Relative size distribution of Blue Sharks for female and male fish from 1998-2011. Vertical lines indicate mean lengths for males (orange) and females (blue).
Off the 1440 Blue Sharks tagged, there were 90 tag returns (6.25%), but this value was skewed by low returns during the later years of the study (1.44%), probably due to the ending of support for the program during the last years and the lag in recaptures. 14.4% of tags were returned from those applied during 1998 and over 10% in 2003 and 2005.
Blue Sharks are habitual wanderers, electronically-tagged fish have been tracked over a total of 29,000 miles in just over 3 years. During this study tags were returned from recaptured fish from as far away as Cape Cod, south of Venezuela and off the coast of equatorial Africa (Figure 4.).
The average time at liberty for tagged sharks was 426 days (24-1913 days) and the average distance from tagging to recapture location was 1894.6 miles (29-4105 miles) giving a range of speeds from
Figure 3 Relative size distribution of Blue Sharks by year for female and male fish from 1998-2011. Vertical lines indicate mean lengths for males (orange) and females (blue).
0.19-15 miles per day. The actual distance travelled per day would be much higher as these figures assume direct travel between locations, an obviously false notion.
Blue Sharks segregate by both sex and age and the recapture data indicates that female fish favour the west coast of Africa, whilst both sexes occupied the ocean between the Azores and the Grand banks (Figure 4.). Due to the rigours of courtship male and female Blue Sharks generally only meet to mate and although the mixing of both sexes occurring in Figure 5. consisted of mature females and immature males, males often practise mating before maturity and analysis of the data showed that both sexes were present at the same time of year, in an area normally dominated my male fish.
Size segregation occurs in Blue Shark populations (Figure 5.) and confirms that during the study period immature female sharks dominated the UK Blue Shark fishery. Previous studies have noted that male sharks tend to range less than females and this study suggests that the male fish seen in UK waters follow the Gulf Stream to our shores, whilst females disperse more and may follow the colder currents south to areas off the West Coast of Africa and maybe ride the equatorial currents to cross the Atlantic (Figure 4.).
Blue Sharks traverse almost unbelievable distances during a year or a lifetime, and that the fish visiting our shores during the summer months could approach the equator by the following January is as miraculous migration as those seen by birds. They favour the warm surface water whilst in high latitudes, but become a creature of the deep when in equatorial seas. Currents and the search for food and prey drive these migrations and larger fish may have both the reserves of fat and a thermoregulatory advantage that size bring to traverse barren areas in search of areas of plenty and mates. So that mature fish tended to travel the furthest is no surprise (Figure 5.)
The environment of the shark also shapes habits and migrations, but the oceans are not constant, changing with the seasons and between years and decades. These rhythms influence the strength and location of currents, temperatures and the other drivers that influence the food chain. Although Blue Sharks swim actively, changes in the environment where they swim still influences their dispersion, so Blue Shark numbers in the UK have always varied with time. However, the added impact of increased commercial pressure distorts these variations and the effects are unpredictable.
Dispersal of sharks is partly a function of stock size. However, Blue Sharks are highly fecund and the population around the UK is currently at a high level. This study suggests that the immature fish which dominate our stock will later disperse widely. If they return is uncertain, but analysis of the data from this study suggests that year classes returned to the UK, and this is especially noticeable in the period from 2008-2010 (Figure 3.).
Unlike terrestrial ecology, its marine cousin finds investigation obscured by the vail of the ocean. Historically anglers and scientist have combined to gather information on marine species and although this relationship seems stretched or fractured, the information provided by studies such as this, still have the capacity to inform scientists and policy makers. Unlike data from commercial fisheries, data derived from recreational angling has a low mortality rate and motivated operators and as this cooperation between the SACGB, skippers and anglers has shown, valuable information which informs conservation is obtainable from simple and cheap tagging studies.
Figure 5 Blue Shark recaptures from the Looe tagging program from 1998-2011 by sex and life history. Red denotes female and black male sharks. Circles denote mature and asterisk immature sharks at recapture. N=73.