Population Parameters & Variable Recruitment in Virginia Tidal River Blue Catfish Populations

Transcription

1 Population Parameters & Variable Recruitment in Virginia Tidal River Blue Catfish Populations Bob Greenlee (84)

2 Pamunkey James Rappahannock Mattaponi Kilometers Sampling Stations Chesapeake Bay Sampling Stations in Four Tidal River Systems 21-28

3 Sampling Strategy Summer Low Frequency (1 pps) Boat Electrofishing Fixed station design Not all rivers sampled each year, and not all stations sampled in each survey Single run per station standardized to 6 s per run after 22 EF boat and Pick-up boat Two netters on each boat Extra netters to assist in landing of unusually large fish (~> 2 Kg) Otoliths Collected for Age & Growth beginning in otolith subsamplingbased on N per cm-group High variability in individual growth => age-length key of limited value After 24, random subsampling Age and Growth analyses based on over, aged individuals

4 Capture Efficiency Low and Variable

5 Unbelievably Abundant One of two tanks collected during 1 minutes of electrofishing on the Rappahannock River

6 Dramatic Differences Among Rivers Sample From Tributary of James

7 Increasing Catch Rates in James and Rappahannock 6 James Pamunkey Mattaponi Rappahanncok r 2 =.99 CPUE (fish/hour) 4 2 r 2 = Survey Year

8 Increase in Upper Limit of Age Structure James River System Frequency (%) James James 22 N (aged) = 33 Age (max) = 13 N (aged) = 216 Age (max) = 1 N (aged) = 33 Age (max) = 19 N (aged) = 468 Age (max) = Age

9 Increase in Upper Limits of Age Structure York System Age Distribution not as developed as James or Rappahannock Mattaponi 1991 YC defines upper limit Pamunkey 1991 or 1992 YC define upper limit Frequency (%) Mattaponi Mattaponi N (aged) = 173 Age (max) = N (aged) = 131 Age (max) = N (aged) = 33 Age (max) = N (aged) = 266 Age (max) = Frequency (%) Pamunkey Pamunkey N (aged) = 148 Age (max) = N (aged) = 192 Age (max) = N (aged) = 327 Age (max ) = N (aged) = 287 Age (max ) = Age Age

10 Growth Rates Differences Among Rivers 22 Comparison (mean total length-at-age of Length Capture w/ 9% of Confidence Blue Catfish Intervals) - 22 (Mean TL +/- 9% C.I.) Length (mm) James Rappahannock Mattaponi Pamunkey Age-Plus

11 Growth Declines in Three of Four Rivers (mean total length-at-age and 9% Confidence Intervals) 12 1 James Mattaponi 2/4 Mattaponi 6/8 Total Length (mm) Pamunkey 22 Pamunkey Rappahannock 22 Rappahannock 2 Total Length (mm) Age Age

12 Declines in Growth Associated With Increases in Density in Pamunkey and Rappahannock 1 Mean Total Length Age-1 (mm) r 2 =.77 r 2 =.99 Pamunkey Rappahannock Log 1 (CPUE +1)

13 Mattaponi A Low Productivity System

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15 Times are Tough Redefining Gape Limitation

16 Growth Rates Differences Among Rivers 28 (mean total length-at-age w/ 9% Confidence Intervals) 12 1 Mean Total Length (mm) James Mattaponi Pamunkey Rappahannock Age

17 Growth Rate James 29 Blue Catfish Mean Weight at Capture (Mean Weight-at-Age w/ 9% Confidence Intervals) (+/- 9% C.I.) 2 Weight (g) 1 1 James Age

18 Growth Patterns and Mortality => Rare Trophy Fish Estimate of Total Annual Mortality Age 8-14 = 9% 3 Weight (pounds) Age

19 Size Distribution James River Blue Catfish Blue Catfish Size Frequency Distribution Tidal James River System - August 26 6 N = Percent of Catch CM Class

20 Temporal Shifts In Size Distributions James James 22 Frequency (%) TL (max) = 113 TL (mean) = 28.9 TL (median) = 26 N = 3,89 TL(max) = 121 TL(mean) = 32.1 TL(median) = 28 N = 1,214 CM-Group

21 3 James 28 3 CPUE (1) = 49 fish/hour Significant Differences Among Rivers in Abundance and Distribution of Quality Size Fish Pamunkey 28 8 Mattaponi Mean CPUE / cm-group = 6.7 f/ishhour Median CPUE / cm-group = 4.2 fish/hour N = 1, CPUE (1) = 86 fish/hour 2 2 Mean CPUE / cm-group = 1.2 fish/hour Median CPUE / cm-group =.4 fish/hour 1 N = 21 1 James CPUE (fish/hour) CPUE (1) = 229 fish/hour Mean CPUE / cm-group = 3.3 fish/hour Median CPUE / cm-group =. fish/hour 1 N = 48 Mattaponi and Pamunkey 1 Rappahannock Rappahannock 27 CPUE (1) = 18 fish/hour Mean CPUE / cm-group =.3 fish/hour Median CPUE / cm-group =. fish/hour 1 N = CM-Group

22 Variable Recruitment 8 James 26 8 Mattaponi Ln Number Z = r 2 =.77 p < Z = r 2 =.39 p < Pamunkey Rappahannock Ln Number Z = r 2 =.1 p < Z = r 2 =.7 p < Age Age

23 James 26 James Year-Class Mattaponi 26 Mattaponi r =.94 p <.1 r =.8 p <.1 Residual Residual

24 Mature Female ~ 17 inches (Age 3 years) 4,, fry per female

25 Conclusions Approximately 3 yrs post-stocking in the James and Rappahannock, and 2 yrs post-stocking in the Mattaponi equilibrium had not been reached for blue catfish populations in Virginia tidal rivers With: Trends of increasing density in the James and Rappahannock Declining growth in the Pamunkey, Mattaponi and Rappahannock Expanding age structure in the James, Pamunkey, and Mattaponi And, increasing size structure in all four populations

26 Conclusions Density of blue catfish in these rivers is extremely high Electrofishing CPUE ranging from 223 to 6,16 fish/hr With trends of increasing density in the James and Rappahannock CPUE in the James and Rappahannock had reached 4,449 fish/hr and6,16 fish/hr respectively by 28.

27 Conclusions Differences in density, size structure, age structure, and growth among rivers were likely due, in large part, to time since population establishment and differences in watershed area and productivity.

28 Conclusions Variable Recruitment In all four rivers, the 1996 year-class was strong and the 1999 year-class was weak, with a weak 22 year-class occurring in three of the four rivers an indication landscape-level environmental processes are involved at some level in determining recruitment in these populations.

29 Conclusions The ecological processes at work in Virginia tidal rivers are generally poorly documented, and information regarding trophic interactions and food web dynamics is lacking. The impact this abundant introduced predator eventually will have on these systems is unknown and may be hard to define, given this lack of information. Further work to address these information gaps is needed.

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31 Conclusions The dramatic increase in population density in the James is of concern, with implications for the trophy fishery it supports. Although no change in growth has been detected to-date in the James, declines in growth associated with increases in density have occurred in thepamunkey and Rappahannock. Until the James population stabilizes (i.e. reaches an equilibrium state) concerns will remain regarding the trophy component of this important recreational fishery.

32 Growth in James River Weight (pounds) yrs = 3 ½lbs 1 yrs = 1 lbs 11 yrs = 2 lbs yrs = 3 lbs Age

33 Growth in Rappahannock River Average Weight of Rappahannock River Blue Catfish at Age yrs = 1 lbs 14 yrs = 2 lbs Weight (pounds) Age

34 Is this to be the fate of the James River population?

35 Vast Increases in Harvest Could be Beneficial If Markets Would Support And Human Health Issues Addressed N = 1,214 Inches Percent of Catch

36 Population Parameters & Variable Recruitment in Virginia Tidal River Blue Catfish Populations Bob Greenlee (84)

37 The 32 Inch Regulation Background Anglers request protection for larger fish Biological information indicates a trophy regulation is warranted High mortality Potential to Recycle fish Virginia Department of Health (VDH) advisory No consumption of blue catfish over 32 inches DGIF contacts VMRC and others regarding impacts on the commercial fishery DGIF staff proposes regulation to Board Biological data VDH advisory 2 Public comment period October, staff recommendation to Board December, Board passes July 1, 26 regulation goes into effect

38 14 12 Blue Catfish Size Distribution Trophy regulation protects less than 1% of the resource Blue Catfish Size Frequency Distribution Tidal James River System - August Percent of Catch N = 111 Inches Trophy fishery multimillion dollar input to regional economy

39 14 Blue Catfish Size Distribution Blue Catfish Size Frequency Distribution Tidal James River System - August 26 Tidal James River Percent of Catch N = 111 Inches Red Box Breeding population not affected by trophy regulation