Winter skate (Leucoraja ocellata) COSEWIC assessment and status report: chapter 9

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Population Sizes and Trends

• Southern Gulf Population
• Eastern Scotian Shelf Population
• Georges Bank-Western Scotian Shelf-Bay of Fundy Population
• Northern Gulf-Newfoundland Population

There are four principal sources of data that can potentially be used to assess changes in winter skate population size: 1) annual fisheries-independent research vessel (RV) surveys conducted for decades by DFO, using standard protocols; 2) non-standard research vessel surveys, conducted prior to 1970, that precede the current standardized surveys; 3) short-duration industry/science surveys that began in the mid-1990s, using either fixed or mobile gear; and 4) information from the directed commercial fishery for winter skate on the Scotian Shelf. Generally, these data indicate number or weight caught, latitude and longitude, depth of capture, time of day, and gear type. Detailed information such as individual length, weight and sex are available for subsets of some surveys (Simon et al., 2003). The sampling areas are delineated by NAFO divisions (Figure 3). Throughout the report, emphasis is placed on the change in abundance of sexually mature winter skate. Declines are estimated as 100*(1-10^(b*t)), where t is the time in years and b is slope of the regression of the log₁₀ of survey CPUE plotted against year.

Southern Gulf Population

Southern Gulf of St. Lawrence Fall RV Survey (Div. 4T)

Data on the Southern Gulf population are available from random-stratified bottom trawl surveys that have been conducted each September since 1971. Comparative fishing experiments did not indicate a change in fishing efficiency when vessels and/or gear changed in 1985 and 1992. More important was the change from day-only fishing before 1985 to 24-hr fishing since 1985. And although winter skate are much more catchable at night than during the day, the abundance index data presented here have been adjusted to account for this change in catchability.

Based on these fisheries-independent survey data, Southern Gulf mature winter skate (>50 cm) are estimated to have declined 98.1% (95% confidence limits = 96.1-99.1%; r²=0.79) from 1971 to 2002 (Figure 5). For all size classes, a decline of 89.3% was estimated for the same time period; this is the same decline estimate reported by Benoît et al. (2003a). The number of winter skate caught per tow (CPUE) was highest in the mid-1970s, declined from a high of 0.6 fish per tow in 1978, and has been below 0.1 fish per tow in 7 of the last 9 years of the survey.

Figure 5: Log Catch Per Unit Effort (CPUE) of Winter Skate (50-74 cm) from DFO Standardized Fall Research Vessel Surveys in the Southern Gulf of St. Lawrence from 1971-2001

The trend line (solid heavy line) = 98.1% decline (r2=0.79); chances are 19 out of 20 that the actual decline is in the range 96.1-99.1% (light lines). Heavy dashed line indicates 50% decline.

Annual length frequencies from the survey (Figure 6) show that there have been fewer large skate (>50 cm) than the long term mean since 1994 (Benoît et al., 2003a; Simon et al., 2003). Of special interest is the fact that no fish over 80 cm have been caught. Since the 1970s, winter skate have experienced a severe truncation in size structure; very few large (>50 cm) individuals are now sampled by the survey (Simon et al., 2003).

Figure 6: The CPUE of Winter Skate from Fall RV Survey Data as Actual Number Per Tow for the Size Classes 0-35, 36-59 and 60-74 cm in the Southern Gulf of St. Lawrence (Div 4T) from 1970 to 2002

Data are from Simon et al. (2003).

The mean percent occurrence of null sets (zero catches) is one indicator of area occupied and suggests how the distribution of winter skate changes over time. Percent occurrence varied a great deal but generally increased during the 1970s and 1980s. It has declined continuously since then to comprise about 3000 km² in 2002 to levels comparable to those observed in the 1970s.

Estimates of minimum total abundance (mature and immature individuals combined) of winter skate have been reported by Simon et al. (2003) (Figure 7). The Southern Gulf total population estimate has ranged from 50 000 to 1 million individuals. The overall average has been 400 000. The decadal averages have been 650, 450 and 170 thousand individuals in the 1970s, 1980s and 1990s, respectively. Current research vessel surveys have estimated the minimum total number of individuals in 2002 to be about 100 000 in the southern Gulf of St. Lawrence (Figure 7).

Figure 7: Total Number of Winter Skate as Estimated by the RV Surveys in Div 4TVWX and 5Z

Note that the 4T survey began in 1971 and the 5Z survey began in 1986. Data are from Simon et al. (2003).

Eastern Scotian Shelf Population

Scotian Shelf Summer RV Survey (Div. 4VWX)

This is the longest running RV survey for winter skate in the Maritimes and is used as one of two primary sources for estimating declines. Other surveys in this area are used for comparison in regards to population trends. Mature winter skate (>75 cm) on the Scotian Shelf have decreased by 91.8% (95% confidence limits = 76.1-97.2%; r²=0.40) from 1970-2003 (Figure 8). Over the same time period the abundance of winter skate of all sizes displays no significant trend.

Figure 8: Log Catch Per Unit Effort (CPUE) of Mature Winter Skate (75 cm+) from DFO’s Scotian Shelf Summer RV Survey (Div. 4VWX) from 1970-2003

The trend line (solid heavy line) = 91.8% decline (r2=0.40); chances are 19 out of 20 that the actual decline is in the range 76.1-97.2% (light lines). Heavy dashed line indicates 50% decline.

The summer research vessel survey has been conducted every July from 1970 to the present (Simon et al., 2003). The sampling gear (bottom trawls) and survey vessel changed in 1982. There were 5621 sets over the survey up to 2002 with winter skate occurring in 815 (14.5%). Catch rates of all skate, including both mature and immature individuals, have varied over the time series, ranging from a high of 2.1 fish per tow in 1979 to a low of 0.3 per tow in 2002. The mean number per tow has declined since the mid-1990s. The 2003 value, however, is near 1.0 per tow and is above the average of the time series.

Composite length frequencies revealed a size range of 9 to 121 cm, with a peak at 49 cm. Catches of skate between 0-35 cm and 36-59 cm have remained stable over the time series. Skate in the size range of 60-74 cm and 75+ cm have shown a steady decline in numbers, reflecting a severe truncation in the size structure of the population (Simon et al., 2003). Eighty percent of the skate caught in the entire survey were smaller than 75 cm in size, the estimated size at maturity for skate in this region; for the time period 1995-2002, 93% of winter skate caught were smaller than 75 cm. Recent trawl surveys have captured only juvenile winter skate in any quantities while adults have been rarely captured (Simon et al., 2003).

The area occupied by winter skate displayed an increasing trend from the 1970s until the early 1990s and has since declined (Simon et al., 2003). The proportion of non-zero sets was highest in 1992 at 21.2% and lowest in 2000 at 8.5%. The composite distribution pattern revealed areas of winter skate concentration, notably in the upper Bay of Fundy, Browns Bank, and the eastern banks and adjoining slopes of the Scotian Shelf. The distribution data decomposed into 4-year time blocks suggest the persistence of concentrations in those areas (Figure 9).

Figure 9: Distribution of Winter Skate (#/tow) from the Scotian Shelf Summer RV Survey (Div 4VWX) for the Years 1970-2002, Aggregated into 4-year Time Blocks

Scotian Shelf Spring RV Survey (Div. 4VWX)

The Spring RV Survey was conducted in March each year from 1979 to 1984. The survey covered the entire Scotian Shelf. Distribution maps aggregated into 1-year time blocks reveal similar areas of concentration as identified in the Summer RV Survey (above) as well as some decrease in area occupied by 1984 (Figure 10). The very limited time period during which this survey was conducted over the entire shelf precludes reliable estimates of decline rate.

Figure 10: Distribution of Winter Skate (#/tow) from the Scotian Shelf Spring RV Survey (Div 4VWX) for the Years 1979-1984, Aggregated into 1-year Time Blocks

4VW Cod March RV Survey (Div. 4VsW)

The survey began in spring 1986 and continues to the present. Data are missing in 1996 (incomplete survey) and 1998 (no survey). With the exception of the anomalously high catch of 20.7 fish per tow in 1994, the rate has never been higher than 6.0 fish per tow (in 1988). The mean number per tow has declined since 1994 to about 0.2 fish per tow, which is below the series average of 3.0 per tow. The estimated decline of mature winter skate (>75 cm) from 1986 to 2001 is 96.4% (90% confidence limits = 49.4-99.2%; r²=0.32) for this survey (Figure 11). Note that this estimate of decline includes the unusually high 1994 datum; the 96.4% decline estimate for mature individuals can, thus, be considered an underestimate. For all size classes, a decline of 90.6% was noted for the time period (90% confidence limits = 50.5-98.5%).

Distribution maps suggest that the area occupied in later years may be smaller than usual (Figure 12). Two distribution indices (area occupied and a metric of population density) suggest a 50% reduction in area occupied since the mid-1980s ((Simon et al., 2003). There is some evidence of a shift in concentration towards deeper slope water although detailed analyses of such a potential shift have not been undertaken.

Figure 11: Log Catch Per Unit Effort (CPUE) of Mature Winter Skate (75 cm+) from the DFO 4VW Cod March RV Survey on the Scotian Shelf from 1986-2002

The trend (solid heavy line) = 96.4% decline (r²=0.32); chances are 1 out of 10 that the actual decline is in the range 49.4-99.2% (light lines). Heavy dashed line indicates a 50% decline.

Figure 12: Distribution of Winter Skate (#/tow) on the Scotian Shelf from the Spring 4VsW Cod Survey, Aggregated into 4-year Time Blocks

The data for 1996 are considered invalid; there was no survey in 1998.

Eastern Scotian Shelf Fall Sentinel Survey (Div. 4VsW)

A sentinel fishery for cod and haddock began in September 1995 in Division 4VsW with winter skate caught as bycatch, and has continued to present. This fisherman-conducted survey used longlines instead of the standard bottom trawl gear used in DFO research vessel surveys. Skate were not identified to species until 1996. The brevity of the survey time series, coupled with an absence of length frequency data (which prevents identification of mature individuals), precludes the use of these data for estimating magnitude of decline (Figure 13). Winter skate catches were concentrated on Sable Island Bank, with a secondary concentration on the eastern shoal of Banquereau Bank (Simon et al., 2003).

Figure 13: Log Catch Per Unit Effort (CPUE) of Winter Skate from the Fall Sentinel Survey from 1996-2001 on the Eastern Scotian Shelf

Scotian Shelf Industry/Science Skate Spring and Fall Surveys (Div. 4VsW)

As part of the Conservation Harvesting Plan for skate established in 1994, the commercial fishing industry agreed to conduct two industry/science surveys per year (spring and fall) on the Scotian Shelf. In 1994, an exploratory skate survey was initiated. DFO specified the fishing locations and the use of 155-mm mesh commercial bottom trawl gear. In 1995, a random-stratified survey design was used with 255-mm mesh gear. The area surveyed was chosen to coincide with the winter skate distribution of the Summer RV Survey (strata 46-58). In 1996, the survey gear reverted back to 155-mm mesh to provide more complete sampling of the size range of the population (Simon et al., 2003). A total of 378 sets were completed in the Spring Survey with 48.2% of the sets catching winter skate. The fall series had 274 sets with 49.6% catching winter skate (Simon et al., 2003).

Winter skate biomass estimates from the Spring Industry survey ranged from a high of 20 500 t in 1996 to a low of 4 900 t in 1997 with the 2002 estimate being 5 600 t (Figure 14). The Fall Industry survey estimates ranged from a low of 9 400 t in 1995 to a high of 47 800 t in 1999. Average biomass estimates since 1995 have been 10 000 t, 20 000 t and 1 300 t from the Spring Industry, Fall Industry and summer RV surveys, respectively. The industry estimates are on the order of 6-12 times greater than the summer RV biomass (which were calculated only for the same strata as those surveyed by industry). This reflects the increased catchability of the rockhopper gear used in this survey as compared to the Yankee 36 and Western IIA trawls used in the Summer RV Survey. This indication of the low catchability of the RV survey gear makes any population estimates derived from that source highly underestimated (Simon et al., 2003). Decadal averages for the Summer RV Survey were calculated to be approximately 6000 t in the 1970s, 3800 t in the 1980s and 1300 t in the 1990s to present. Of note are the extremely low estimates in 2001 and 2002 from the Summer RV Survey (200 t and 50 t, respectively). The Industry Survey suggests there has been little change in biomass since 1996 (Simon et al., 2003).

Figure 14: Total Biomass (tonnes) of Winter Skate as Calculated for Strata 46-58 of the Scotian Shelf from the Stratified Spring (1995-2002) and Fall (1995-1999) Industry Surveys

Stratified length frequency data were not collected during these surveys, precluding the estimation of abundance of mature winter skate. The Spring Industry Survey caught most fish on the edges of Banquereau Bank and the eastern Scotian Shelf. The Fall Survey catches exhibited a similar pattern, but in addition there was a concentration of skate west of Sable Island (Simon et al., 2003).

Scotian Shelf Commercial Fishery

A small developmental directed commercial fishery for skate by Canada began in 1994 on the Eastern Scotian Shelf (Div. 4VsW) with a total allowable catch (TAC) of 2000 t allocated to four otter trawlers. Landings by that directed fishery declined from 2152 t in 1994 to less than 400 t in 2001, an 81% decline reflecting progressive reductions in TAC. Reductions in TAC arise from concerns about the impact of the directed fishery on stock status and have resulted in a reduction of in the number of vessels prosecuting the fishery to 3 in 2001 and 1 in 2002. Landings have been primarily winter skate (>90%) with the majority of the fishery centred on Banquereau Bank (Simon et al., 2003). Average total landings for the years 1980-1993 when the fishery was open to all countries were about 1000 t annually (Simon and Frank, 2000).

Discards of winter skate by groundfish fisheries prior to closures in 1994 were estimated to be greater than 1000 t. Since 1994, discards were estimated to have been generally less than 100 t (Simon et al., 2003), although it should be recognized that this estimate is highly uncertain.

Commercial sampling of winter skate began in 1995. The length frequency peaked in 1995 at 76 cm and included many fish up to and greater than 100 cm. In 1996, the peak shifted to 71 cm and remained there to 2002. This is slightly below the estimated size at maturity of 75 cm.

The percentage of fish greater than 90 cm declined from 25% in 1995 to 6% in 1996 and less than 4% since 1997 (Simon et al., 2003). Length frequency observations by the International Observer Program (IOP) from the commercial phase of the directed fishery showed a similar loss of larger individuals from the population as the commercial sampling, but there was some indication of improvement in the percentage of numbers of fish greater than 76 cm. The percentage of fish greater than 90 cm decreased from 22% in 1995 to less than 3% from 1996-2000. In 2001 and 2002, the percentage increased to 4 and 5%, respectively (Simon et al., 2003).

The summer research vessel survey abundance estimate for the entire Scotian Shelf indicated that the minimum population numbers peaked in the early to mid-1990s and have been below the long-term average of 2.6 million up to 2002. The dynamic range of the data has been from 8 million in 1979 to 750 000 in 2001. The 4VW Cod Survey showed that total numbers of winter skate have averaged about 4.8 million individuals, excluding the high estimate in 1994 of 43 million individuals. The summer RV estimate for the comparable area averaged 1.7 million individuals. In the last 11 years, the mean spring RV estimate was 3 million individuals while the summer RV estimate was 1.5 million (Simon et al., 2003).

In 2002, the minimum estimate of the total number of individuals on the eastern Scotian Shelf (4VW), based on data obtained from the Summer RV survey data, was approximately 750 000 individuals (Figure 7).

Georges Bank-Western Scotian Shelf-Bay of Fundy Population

Scotian Shelf Summer RV Survey (Div. 4VWX) and Spring RV Survey (Div. 4VWX)

The composite distribution pattern from both surveys revealed areas of winter skate concentration in the Bay of Fundy and on Browns Bank off southwestern Nova Scotia. The distribution data decomposed into 4-year time blocks suggest stability in these concentrations in those areas over the years for which data are available (Figure 9). Estimates of the minimum abundance of the total population size of winter skate in NAFO division 4X have remained relatively stable since the early 1970s (Figure 7).

Georges BankWinter RV Survey (Div. 5Z)

Based on data collected during the Georges Bank (Div 5Z) Winter RV Survey, the abundance of mature winter skate (>75 cm) declined from 1986 through 1994, but exhibited a general increase thereafter to 2004 (Figure 15). Given the long generation times that have been estimated for winter skate in this area (22 years), the comparatively short time period over which these changes in abundance were observed suggests that these reductions and increases in abundance might be attributable to small-scale distributional shifts by winter skate into and out of Canadian waters on Georges Bank.

Figure 15: Log Catch Per Unit Effort (CPUE) of Mature Winter Skate (75 cm+) from DFO Standardized Winter Research Vessel Surveys Conducted on the Canadian Portion of Georges Bank from 1986 to 2002

Note: The log (CPUE) data for 2003 and 2004, although not shown in this figure, were between 0 and 0.2.

The Georges Bank survey began in 1984 when a partial survey was conducted. In 1986, the survey was expanded to cover both the Canadian and U.S. portions of the bank (Simon et al., 2003). A total of 1556 sets have been completed with 1098 (70.5%) containing winter skate. The number per tow (Figure 16) has been variable, with a mean catch rate of 18.1 fish per tow.

Figure 16: Mean Number and Weight Per Tow of Winter Skate in the Georges Bank Winter RV Survey for the Years 1986-2002

Composite length frequencies showed a size range from 9-112 cm, with a peak at 55 cm. Catch rates for winter skate <35 cm and 36-59 cm have been variable over the time series with only a slight decrease. A decline in the proportional representation of skate greater than 75 cm to 1994 was followed by an increase thereafter to 2004.

Distribution maps indicate that winter skate are found throughout Georges Bank and that the area occupied seems to have undergone little change from 1986 to 2000. No trend in overall winter skate number per tow is evident, though the index has risen in 2004 to 37.3, which is well above the series mean of 19.2 fish per tow.

The Georges Bank RV survey (winter) revealed overall population numbers ranging from 6.9 million in 1994 to 48 million in 1990. When considering only the Canadian portion of the population, numbers have ranged from 600 000 to 3.8 million and average 1.8 million. No trend in the data was evident (Simon et al., 2003).

In 2002, the minimum estimates of the total number of individuals in NAFO divisions 4X and 5Ze were approximately 500 000 and 1.2 million, respectively (Figure 7), resulting in a minimum estimate of total abundance of 1.7 million for winter skate in this Designatable Unit.

Status of Winter Skate in U.S. Waters

Until 2000, the U.S. population of winter skate was considered to be in an overfished state. However, its status has been changed such that it is no longer considered to be in an overfished condition (NMFS 2002). In its 2002 report to Congress, NMFS (2002) reported that the most recent survey index for winter skate indicated that the current biomass was above the minimum stock size threshold and that winter skate were now officially listed as “not overfished”. This status for winter skate was reaffirmed by NMFS in its 2003 report to Congress (NMFS 2003).

Although winter skate are no longer considered overfished in U.S. waters, winter skate remain at comparatively low levels of abundance. In accordance with the U.S. Magnuson-Stevens Act, as amended by the Sustainable Fisheries Act (1996), winter skate is defined as being in an overfished condition when the 3-year moving average of the NEFSC (Northeast Fisheries Science Center) autumn survey mean weight per tow is less than one-half of the 75^th percentile of the mean weight per tow observed in the autumn trawl survey from 1967 to 1998 (NOAA 2003a). In 2000, although winter skate on Georges Bank was at about the same level of abundance as it was in the early 1970s, it was only 25% of the peak levels of abundance observed in the mid-1980s (NOAA 2003b).

Given the comparatively low level of abundance of winter skate on Georges Bank, the probability that winter skate will immigrate into the Eastern Scotian Shelf population in the foreseeable future is considered to be very small. Thus, the probability of rescue from the U.S. population of winter skate is considered to be very low for the Eastern Scotian Shelf population and negligible for the Southern Gulf population.

Northern Gulf-Newfoundland Population

Newfoundland/Quebec Survey Data (Div. 3KLMNOP4RS)

Winter skate are not and have never been common in Newfoundland waters (Simon et al., 2003). While the survey information available in this region is the most temporally extensive, the earlier years in the series (pre-1978) were not associated with any standard survey design. A random-stratified survey design was adopted in 1970, 1971 and 1978 in divisions 3LNO, 3P and 2J3K, respectively; 58 677 sets were completed from 1947 to 2002 with winter skate occurring in only 240 (0.4%) of them (Simon et al., 2003).

In the winter and summer surveys of the northern Gulf of St. Lawrence (Div. 3Pn4Rs), winter skate were taken in only 33 of 3,615 sets (0.9%) from 1947 to 2002. Industry sentinel fishery surveys have been conducted in the northern Gulf in July and October annually since 1995 (Simons et al., 2003). Of the 14 surveys that have been conducted to date, only 5 winter skate have been caught.