Shortfin mako (Isurus oxyrinchus) COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

Information sources

Atlantic population

All catch information, including distribution of fishing effort, is from the Maritimes Region and Newfoundland International Observer Programs (IOP), sport fishing derby data, the International Commission for the Conservation of Atlantic Tunas (ICCAT), and the Department of Fisheries and Oceans (DFO) catch statistics databases. Data used to develop a catch rate model is from observer and logbook data. There has been no effort to specifically survey for shortfin mako in Canadian waters; therefore all population and trend data are derived from fisheries data.

Abundance

There are no reliable population-level stock assessments available for shortfin mako anywhere in the North Atlantic due to severe limitations in the quantity and quality of data available (ICCAT 2005a). Shortfin makos are considered bycatch in pelagic longline fisheries with the swordfish fishery being the main source. An analysis of observed sets in the Maritimes Region between 1990-2003 indicates that mako typically make up less than 2-3% of the catch by weight (Campana et al. 2004; Campana et al. 2005). Due to the high value of the meat, discard rates are very low and therefore landings are similar to the nominal catch.

In Canada, landings reported from Canadian vessels have declined since 1994 from 142 t to about 66 t in 2003 (Figure 7; Table 1). However, this decline is almost certainly a result of misreporting porbeagle as shortfin mako prior to 1996 due to a data coding issue, where porbeagle and shortfin mako were coded together under the heading of mackerel sharks. Since 1996 landings have been relatively constant but total effort is unknown. In addition to Canadian landings, between 1980 and 1999 there was a small international fishery operating in Canada’s Atlantic waters, comprised of Japanese and Faroese vessels with total landings of 163 t(~8.1 t/yr) (Table 1). Since 1999, there have been no international fisheries reporting shortfin mako in Canadian waters. Derby/recreational catches of shortfin makos are minor with only a few sharks landed per year (Campana et al. 2004).

Landings in the entire North Atlantic are from ICCAT (2005b) representing all nations and all regions. Nominal annual recorded catches/landings have averaged approximately 2,800 t since 1997 (Table 1). Landings prior to 1997 are underreported, primarily due to the absence of data from the Spanish fleets (ICCAT 2005b). In the northwest Atlantic (north of Florida), average landings since 1997 have been 1,100 t (Table 1).

Figure 7. Atlantic Canada landings of shortfin mako by fishery between 1994-2003. Data from DFO Zonal Statistics File except for 2003 which is from MARFIS.

Figure 7.  Atlantic Canada landings of shortfin mako by fishery between 1994-2003. Data from DFO Zonal Statistics File except for 2003 which is from MARFIS.

Table 1. Reported shortfin mako shark landings (t) by country in the North Atlantic.

Note: For accessibility reasons, Table 1 has been split in three sections: Canadian Atlantic (NAFO Areas 2-5); Northwest Atlantic; and North Atlantic.

Table 1. Reported shortfin mako shark landings (t) by country in the Canadian Atlantic (NAFO Areas 2-5)
Year Canada Faroe Island Japan Other Total
1979
 
 
0
 
0
1980
 
2
0
 
2
1981
 
 
1
 
1
1982
 
 
0
 
0
1983
 
 
5
 
5
1984
 
 
1
 
1
1985
 
 
 
 
 
1986
 
 
2
 
2
1987
 
 
10
 
10
1988
 
0
17
 
18
1989
 
1
13
 
14
1990
 
5
8
 
13
1991
 
2
14
 
16
1992
 
2
29
 
31
1993
4
0
16
 
20
1994
142
 
21
 
164
1995
111
 
4
 
115
1996
67
 
5
 
72
1997
110
 
2
 
111
1998
71
 
1
0
72
1999
70
 
2
 
72
2000
79
 
 
 
79
2001
70
 
 
 
70
2002
79
 
 
1
79
2003
66
 
 
 
66

 

Table 1b. Reported shortfin mako shark landings (t) by country in the Northwest Atlantic
Year Japan USA Other Total
1979
 
 
 
 
1980
 
 
 
 
1981
 
 
 
 
1982
226
 
 
226
1983
85
 
 
85
1984
213
 
 
213
1985
214
 
 
214
1986
231
 
 
231
1987
232
 
 
232
1988
168
 
 
168
1989
176
 
 
176
1990
140
 
 
140
1991
198
 
 
198
1992
345
 
 
345
1993
553
236
 
789
1994
450
273
 
723
1995
397
230
 
627
1996
238
181
72
491
1997
99
213
808
1120
1998
107
0
999
1106
1999
123
0
969
1092
2000
83
0
785
868
2001
116
160
637
913
2002
 
154
1066
1220
2003
 
128
1277
1405

 

Table 1c. Total Reported shortfin mako shark landings (t) in the North Atlantic
Year Total
1979
 
1980
 
1981
 
1982
 
1983
 
1984
 
1985
 
1986
 
1987
 
1988
 
1989
 
1990
193
1991
314
1992
246
1993
1094
1994
978
1995
1088
1996
1136
1997
3307
1998
2586
1999
2439
2000
2144
2001
2360
2002
2918
2003
3801

Canadian Atlantic landings

From DFO Zonal Statistics File, except for 2003 which is from MARFIS; Japan, Faroes, other countries in Canadian Atlantic are from Maritimes Region & NF IOP (excludes discards).

NW Atlantic landings

Japan in NW Atlantic represents nominal catch of unspecified sharks and rays from FAO Statistics (2001). U.S. and other countries are from ICCAT statistics for Areas 1 (NW Atlantic) and 39 (Northwest Central Atlantic). Note, some countries do not report at a regional scale.

Total North Atlantic landings

From ICCAT statistics for nominal catches of sharks (1990-2003). Includes all countries with reported landings from the North Atlantic including Canada (ICCAT 2005b). ICCAT Areas: ATL, ATMED, AZOR, GOFM, MEDI, NE, NW, NWC, WTRO

Campana et al. (2004) report that international landing statistics for shortfin makos are probably underreported. If this is true, and if it is also true that the estimates of Canadian catch are more accurate, then estimates of the fraction of total catches that are taken in Canadian waters are too high. Since 1997, total reported catch in Canadian Atlantic waters has been 549 t compared to 7,724 t in the northwest Atlantic (~7%) and 19,554 t in the entire North Atlantic (2.8%). Because some countries do not report catches by regional areas, the most accurate comparison of landings between countries is at the scale of the entire North Atlantic.

Although an estimate of the proportions of mature animals in Canada’s Atlantic waters has been derived (see Lifecycle and Reproduction section), there are no reliable means for estimating the total abundance of mature individuals in the Atlantic Ocean, as the total population in Canadian Atlantic waters is unknown.

Fluctuations and trends

There are no surveys specifically designed to assess shortfin mako trends in the North Atlantic or elsewhere in Canada.. Despite the general lack of shortfin mako research, there have been some attempts to examine trends based on data collected from various pelagic fisheries. In the following paragraphs, four of the most recent and relevant analyses are summarized to provide insight into the current trend.

Baum et al. (2003), using U.S. pelagic longline logbook data, found that the catch rates of makos (thought to be mostly shortfin mako) in the northwest Atlantic had declined by 40.7% over the time series (Figure 8). The Baum et al. (2003) analysis was based on 65,795 makos caught by 214,234 longline sets (mean of 550 hooks/set) between 1986 and 2000. Data prior to 1986 did not accurately account for individual shark species and therefore was not used in their analysis. The index of abundance was based on a standardized catch per unit effort (CPUE) time series for area, season, fishery variables, and year. Aspects of the analysis by Baum et al. (2003) were recently questioned by several U.S. shark scientists who suggested that the trend observed for the shortfin mako should be considered preliminary without the full benefit of data from multiple international sources and a complete stock assessment (Burgess et al. 2005).

Figure 8. A. Relative abundance of shortfin mako in the entire west Atlantic indicated by an analysis of U.S. commercial longline fishery logbook data from 1986-2000 (decline of 40.7%);
B. Estimated annual rate of change for nine assessment areas and total. From Baum et al. 2003.

Figure 8. A. Relative abundance of shortfin mako in the entire west Atlantic indicated by an analysis of U.S. commercial longline fishery logbook data from 1986-2000 (decline of 40.7%); (B) estimated annual rate of change for nine assessment areas and total. From Baum et al. 2003.

A large-scale investigation into shortfin mako trends in the North Atlantic was recently undertaken by ICCAT (ICCAT 2005a). This study concluded that shortfin mako have likely experienced some level of stock depletion in the North Atlantic based on declines in catch rates of 50% or more from 1971-2003 (Figure 9). Catch rates used in the analysis were from the Japanese longline (1971-2003) and U.S. longline logbook data (1986-2003). Modelling results from the same report indicate that the ratio of the current level of spawning stock biomass relative to virgin conditions (SSB/SSB0) is 0.31-0.33 (ICCAT 2005a). Due to limitations on quantity and quality of the information available for the stock assessment of shortfin mako, the assessment is described as very preliminary (ICCAT 2005a). It should be noted that catch rates from the U.S. longline logbook data used in the ICCAT analysis suggest a small upswing between 2000-2003 (Figure 9).

Figure 9. Indexed CPUE of shortfin mako in the North Atlantic from the Japanese longline fleet (JLL) and United States longline fleet (USLL). Index achieved by identifying the years common to all indices, calculating the mean within each series for those common years, and expressing all values in each series as a proportion of the calculated mean for that series. Figure from ICCAT 2005a.

Figure 9. Indexed CPUE of shortfin mako in the North Atlantic from the Japanese longline fleet (JLL) and United Stateslongline fleet (USLL).

Research into shortfin mako trends in exclusively Canadian Atlantic waters is limited to a single study by Campana et al. (2004). In this study the authors developed a catch rate model using logbook data representing longline sets between 1988 and 2003 where at least one mako was recorded in a trip. There was no observed decline in catch rates over the time series (Figure 10). However, due to small sample sizes and large variances, the analysis was of low statistical power and would have been unable to detect anything other than a severe decline (or increase); in addition this index represents part of the overall population found in Canadian Atlantic waters and could have been affected by factors related to migration rate into this area.

The median size of shortfin mako in the Canadian Atlantic catch was used as an additional indicator of population status, where a long-term change in the median size may signal the degree of exploitation (Campana et al. 2004; 2005). A plot of median fork length against year of collection indicated a gradually increasing trend in the Japanese fishery until 1996 (Figure 11). However, median size in the Canadian Atlantic fishery declined between 1998 and 2003. Since there is no overlap in the time series of the two countries (due to insufficient or absent observer effort in some years), it is difficult to determine if the disparate trends reflect targeting of different groups of shortfin mako or a real change in trend. However, the recent decline in the mean size in the Canadian fishery suggests a decline in abundance of larger shortfin makos (since there is no evidence of an increase in abundance of smaller sharks based on length frequency data).

Figure 10. Standardized trip-level catch rate of the weight of shortfin mako sharks caught by pelagic longliners on the Scotian Shelf between 1988 and 2003. Data were restricted to Japanese longliners targeting bigeye tuna between Oct-Dec of 1987-99 and Canadian longliners targeting swordfish between July-Sept of 1996-2003. The GLM model was fit to non-zero trips using a gamma error distribution and with Year and Vessel as factors. Error bars represent 1 SE around the mean. There was no trend in catch rate through the time series. Source: Campana et al. 2004.

Figure 10. Standardized trip-level catch rate of the weight of shortfin mako sharks caught by pelagic longliners on the Scotian Shelf between 1988 and 2003.

Figure 11. Long-term changes in the mean fork length of shortfin makos caught by Japanese (solid squares) and Canadian Atlantic (open circles) pelagic longliners. Error bars represent 95% confidence intervals around the mean. In both the Japanese and Atlantic Canadian fishery, observer coverage was sporadic in some years, resulting in fewer than 10 length measurements per year; these data are not presented, resulting in non-overlap of the two time series.

Figure 11. Long-term changes in the mean fork length of shortfin makos caught by Japanese (solid squares) and Canadian Atlantic (open circles) pelagic longliners.

In summary, four indices of population trends are shown, two from international waters and two from Canadian waters. Both studies from international waters suggest shortfin mako populations in the North Atlantichave likely undergone some level of depletion based on declines in catch rates. The trend presented by Baum et al. (2003) is the most reliable trend presently available to assess the status of shortfin mako at a large spatial scale because of its use of a single consistent data set (US longline fleet logbooks) and analytical methods. Confidence in the ICCAT series is weaker because of possible misidentification issues in years prior to 1995. Canadian indices consist of a catch rate model and a trend in median fork length. The catch rate time series from Canadian waters does not indicate any trend. Interpretation of biological data collected in Canadian waters is constrained by years of insufficient or absent data, but a recent decline in median fork length is apparent.

Because shortfin mako in Canadian Atlantic waters represent the margins of their distribution, trends derived from these data may be influenced by several ecological factors (e.g., ocean temperatures, prey abundance) that may naturally shift the abundance in Canadian waters over varying temporal and spatial scales. For this reason, analyses representing a larger spatial scale present a more accurate picture of the population as a whole.

Rescue effect

Canada’s Atlantic waters represent a small portion of shortfin mako habitat. Population status and rescue effect at any given time in Canadian Atlantic waters depends heavily on the status of the population as a whole. If the ocean basin-wide population increases, so should the relative abundance in Canadian Atlantic waters and vice versa. Furthermore, this species distribution is largely influenced by temperature and therefore one would expect thermal oceanographic patterns to influence the relative abundance in Canadian Atlantic waters.

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