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Author Topic: Bay-Wide Winter Dredge Survey-2003/Part 1  (Read 2796 times)
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Crabpop
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« on: December 11, 2003, 09:50:53 PM »

OK, guys and gals, the Chesapeake Bay-Wide Blue Crab Winter Dredge Survey is out and it actually contains some good news.  I haven't read the entire thing yet, but the abundance of Blue Crabs that winter-over has again shown a positive increase.  Ain't what it used to be, but it's movin' in the right direction!  Read the report below, or mouse the link below to read the entire report, complete with graphs.  Hey, Spring's right around the corner! Grin

P.S. The report's too long to post in a single topic, so had to post in two parts.

Crabpop

http://www.dnr.state.md.us/fisheries/crab/winter_dredge.html

INTRODUCTION: The winter dredge survey is the only Bay-wide fishery-independent effort to assess status of the blue crab population in the Chesapeake Bay. The dredge survey produces estimates of abundance, indices of recruitment, indices of female spawning potential, and estimates of relative exploitation of the blue crab by commercial and recreational fisheries. A pilot version of the study was first conducted in 1988 with the cooperation of the University of Maryland Chesapeake Biological Lab (CBL). In 1989 the Virginia Institute of Marine Science (VIMS) joined the survey, and the two states continue to sample each winter from December through March.  
   

 SAMPLING AREA AND INTENSITY: Since 1994, the survey has been conducted according to a stratified random design (Rothschild and Sharov, 1996). The Bay is divided into three strata: Lower Bay (the mouth of the Chesapeake to Wolf Trap Light, Middle Bay (Wolf Trap Light to Cove Point) and the Upper Bay/Tributaries (Cove Point to Pooles Island and all of the Bay’s tributaries). A total of 1500 sites in waters deeper than 1.5 m (5 ft) are randomly selected each year. The number of sites assigned per stratum is proportional to the area of that stratum. In addition to the comprehensive coverage of the Bay using stratified random sampling, 125 fixed sites are also sampled by MDNR and VIMS as part of the survey.
   
SAMPLING PROTOCOL: A 1.83 m wide Virginia crab dredge fitted with a 1.3 cm (0.5 in) nylon mesh liner is towed along the bottom for one minute at a speed of three knots. Latitude and longitude, measured with a Differential Global Positioning System (GPS) are recorded at the beginning and end of each tow to determine distance covered.  Distance is multiplied by the dredge’s width to calculate the area covered. Beginning and ending depth, water temperature, and salinity are recorded at each site. Blue crab carapace width (CW) and weight are measured (to the nearest millimeter and gram). Sex is determined and maturity of females is noted.
Each year gear efficiency is estimated by dredging repeatedly in areas of medium or high crab density.  Each experiment consists of six coverages of a 100m X 5.5m (three dredge-width) area. One coverage is considered to be 3 adjacent, non-overlapping tows within the experiment area.  The decline of crabs in each consecutive coverage allows us to estimate the efficiency of the dredge and to correct the final estimates of abundance accordingly. For more information see: Volstad et al. 2000 and Sharov et al. 2001.
 

    Results    
     
Age Determination Mean Crab Density Relative Abundance  
Blue Crab Size Relative Exploitation Rates Absolute Abundance
 AGE DETERMINATION:
The blue crab population is made up of predominantly three age classes; age 0, age 1, age 2+. The age classes are separated by their carapace width or maturity as listed below. This allows us to analyze density of crabs that were entering two important fisheries, peelers (age 1) and hard crabs (age 2).
 
Code Maturity Level
 Size Criteria
 
(M0) Age 0 Males Males < 60mm CW
(F0) Age 0 Females Females < 60mm CW
(M1) Size 1 Males Males 60-119mm CW
(F1) Size 1 Females Immature females &#8805; 60mm CW
(M2) Size 2 Males Males 120mm &#8805; CW
(F2) Mature Females Mature Females
(M1+) Age 1+ Males Males &#8805; 60mm CW
(F1+) Age 1+ Females Females &#8805; 60mm CW
(T0) All age 0 M0 + F0
(T1) All size 1 M1 + F1
(T2) All size 2 Males Mature Females M2 + F2
(T1+) All age 1+ 1 + F1 + M2 + F2
   
   
MEAN CRAB DENSITY:
Mean crab density (D) is calculated with the following equation, where D is measured in units of crabs per 1000m2 (Cochran, 1977). The equation is used to determine the amount of crabs that are in each of the three strata.
D = &#8721; (ah/at (&#8721;(ci/ai)h/nh)) x 1000
 Where:

     ah = area, in km2, of stratum h,

     at  =   total area of all strata combined,

     ai  =    area covered by dredge tow I,

     ci  =    number of crabs captured in dredge tow I, and

     nh = number of dredge towns in stratum h.
 
 
Total mean crab densities are calculated for every year of the winter dredge survey. The table below presents total crab densities for years 1990 to 2003. Use the “Code” abbreviations listed in the table in Age Determination as a key for the age of the blue crabs in the mean crab density table below (T = Total Crabs/all age classes). The mean crab density is measured in the number of crabs per 1000m2, so the mean abundance of total crabs (T) in 1990 is 82.16 crabs per 1000m2.

 

Bay-Wide mean blue crab densities for each age category
Year T T0 T1 T1+ T2 M0 F0 M1 F1 M2 F2
1990 82.16 47.34 16.97 34.82 17.85 26.53 21.99 11.69 5.89 8.84 9.01
1991 85.53 36.43 18.38 49.11 30.73 20.13 19.41 13.61 6.04 12.14 18.59
1992 38.14 10.69 7.06 27.44 20.38 4.98 5.54 4.34 2.73 4.86 15.52
1993 88.32 51.27 24.63 37.05 12.42 25.03 25.05 11.95 8.56 6.21 6.21
1994 53.55 30.94 8.77 22.61 13.84 15.38 15.83 6.08 2.85 4.56 9.28
1995 50.43 30.71 11.35 19.72 8.37 13.13 15.37 6.24 3.75 4.01 4.36
1996 76.50 51.82 9.31 24.68 15.37 25.37 26.53 5.60 3.71 5.30 10.07
1997 69.76 52.16 6.71 17.59 10.88 24.01 27.97 3.95 2.73 3.40 7.48
1998 36.43 16.87 7.38 19.54 12.16 7.54 9.34 4.40 2.98 3.89 8.27
1999 31.93 22.76 2.43 9.17 6.74 9.58 13.19 1.21 1.21 2.01 4.73
2000 30.49 13.90 3.77 16.59 12.82 6.45 7.44 2.31 1.47 2.11 10.71
2001 26.64 15.86 4.11 10.76 5.70 7.05 7.40 2.65 1.96 1.93 4.24
2002 32.35 19.81 3.68 12.54 7.29 7.25 9.79 2.85 1.47 3.82 4.40
2003 39.80 17.88 9.32 21.91 12.59 9.53 8.35 5.97 3.35 7.90 7.69

RELATIVE ABUNDANCE:
All age classes: Relative abundance has been increasing since 2001, but remains well below abundance totals seen prior to 1998 (Figure 1). When regressed against commercial landing data, density shows a significant, positive relationship with amount landed (Figure 2).
 

Figure 1: Density of blue crabs estimated from the Chesapeake Bay winter dredge survey, 1990-2003, with 95% confidence intervals.
 
 

 Figure 2: Linear Relationship of estimated over-wintering blue crab density and Bay-wide commercial harvest. Each block represents a year.
Age 0 and Age 1+: Age 0 crab densities have shown no trend from 1998 to present (Figure 3). Recruitment was relatively high prior to 1998 with the exception of poor recruitment in 1992. Since 1998 recruitment has shown no significant variation and has been poor. For the first time since 1993 the 2003 relative abundance of Age 1+ male and female crabs increased significantly, marking the first upswing in a trend that has been steadily declining since 1993. Age 1+ female densities have fluctuated without trends. The 2003 density estimate is significantly higher then the 2002 estimate, the second lowest annual estimate in the survey (Figure 5). Density of adult males (>120mm CW) has declined over time. The 2003 estimate is higher then the estimate reported in 2002 (Figure 6).

Figure 3: Density of age 0 blue crab (<60mm CW) estimated from the Chesapeake Bay winter dredge survey, 1990-2003, with 95% confidence intervals
 


 Figure 4: Density of age 1+ blue crabs (immature females 60+ mm CW and males 60-120 mm CW) estimated from the Chesapeake Bay winter dredge survey 1990-2003, with 95% confidence intervals  

Figure 5: Density of mature female blue crabs estimated from the Chesapeake Bay winter dredge survey, 1990-2003, with 95% confidence intervals
 


 Figure 6: Density of mature males (&#8805; 120mm CW) blue crabs, adjusted for catchability, estimated from the Chesapeake Bay winter dredge survey, 1990-2003 with 95% confidence intervals.

(Continued in Part 2)
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« Reply #1 on: December 11, 2003, 09:54:21 PM »

BLUE CRAB SIZE:
The mean size of exploitable hard crabs (mature females, legal males = males > 127mm CW) had not shown any trends or significant fluctuations over the duration of the study. Mean size of males in 2003 is significantly lower then the record high in 2002 (Figure 7 and Figure 8 ).

Figure 7: Mean carapace width of mature female blue crabs captured in the Bay-wide winter dredge survey, with 95% confidence intervals.
 


 Figure 8: Mean carapace width of legal male blue crabs (+127mm CW) captured in the Bay-wide winter dredge survey with 95% confidence intervals.

RELATIVE EXPLOITATION RATES:
Relative exploitation rates are an estimate of the number of crabs being removed from the population by harvesting. Relative exploitation rates have almost doubled since 1990. There has been a statistically significant increase in the exploitation rates over the time series (Figure 9).

Figure 9: Estimates of relative exploitation rate, based on reported harvest and on the Bay-wide winter dredge survey population estimates.
 

ESTIMATED ABSOLUTE ABUNDANCE:
Estimated absolute abundance is the estimate of how many blue crabs are over-wintering in the Chesapeake Bay. The number is based on the density of crabs (crabs/m2) but multiplied over the entire sample area (9814.57 km2). The estimated for each year are listed in Table 5:

Table 5. Annual estimates of over-wintering abundance (in numbers) of blue crabs in Chesapeake Bay, 1990 – 2002.
Year Abundance
1990 806,365,071
1991 839,440,172
1992 374,327,700
1993 866,822,822
1994 525,570,224
1995 494,948,765
1996 750,814,605
1997 684,664,403
1998 357,446,639
1999 313,379,220
2000 299,246,239
2001 261,460,145
2002 317,501,340
2003 390,619,886


References:
Cochran, W.G. 1977. Sampling Techniques. John Wiley and Sons. New York. 428 p.

Davis, G.R., B.K. Davis, J.C. Walstrum. 2002. Population dynamics and stock assessment of the blue crab (Callinectes sapidus), in Chesapeake Bay. MDDNR Winter Dredge Annual Report 2002.

Rothschild, B.J. and A.F. Sharov. 1996. Abundance estimation and population dynamics of the blue crab in the Chesapeake Bay: final report to NOAA Chesapeake Bay stock assessment committee. UMCEES [CBL] 95-207.

Sharov, A.F., J.H.Volstad, G.R.Davis, B.K. Davis, R.N. Lipcius and M. Montane. In press. Abundance and exploitation rate of the blue crab (Callinectes sapidus) in Chesapeake Bay. Bull. Mar. Sci.

Glossary:
Carapace width: The width of the crab measured from point to point.

Recruit: In this case, newly settled juvenile crabs, but also refers to crabs that have moved into a certain size or age class. Male crabs recruit to the fishery at 5.25 inches.

Recruitment: A measure of the number of crabs that enter the population within the year. Also a measure of the number of crabs that grow to harvestable size within the year.

Relative exploitation: An index of the fraction of crabs removed by harvest over time.

Spawning potential: The number of eggs that could be produced by an average recruit.

Stratified random design: An experimental design that separates the bay into three geographic areas (strata), choosing a number of random sites, proportional to the area of the strata.

Virginia Crab Dredge: A dredge consisting of a metal frame with a bag net made of iron rings, s-hooks, or nylon netting. The leading edge of the device is a heavy iron bar with 5-7 inch teeth. Comes in a variety of sizes and dimensions but is typically 6-8 feet wide and weighs around 250 pounds.


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Date last updated: 12/08/2003
« Last Edit: December 11, 2003, 10:05:52 PM by Crabpop » Logged

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Qcrab
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« Reply #2 on: December 12, 2003, 06:16:12 PM »

Hey Crabpop

This is very interesting. After reading all of the info I have one question...

What are those little black things in fruitcake that look like bits of tire?

Qcrab
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« Reply #3 on: December 12, 2003, 06:52:21 PM »

Qcrab, those little black things are, in fact, little pieces of tire.  The larger pieces are used as a safety surface for childrens playgrounds.  This is why I don't eat fruitcake...

Crabpop  
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Qcrab
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« Reply #4 on: December 12, 2003, 06:57:07 PM »

Pop!

LOL

Your the best! Happy Happy Holidays.

Qcrab
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« Reply #5 on: December 13, 2003, 02:03:25 PM »

Duhh  who is waisting money on this survey! I'm no scientist but In lay terms, it would seem a no brainer that dredging for crabs should be banned, (at the very least for the off season) for example, if the entire population of crabs in the bay was 100 then you dredge 1/2 of them while they hibernate over winter, then next year you will only have 50 to get things going, and at some point you will have no crabs left, thats why they have seasons on hunting and other things like that, hey wouldn't it be great if we could hunt animals while they were hibernating over a season, or grow tomatoes in december, but we live in a seasonal area of the country and cannot! These big commerical crabbing outfits are doing just that by dredging, they are taking a big bite out of next years harvest, leaving less and less crabs for the natual repopulation process, does that not make sense if they are taking all these males and females by dredging that come spring there will be that many less crabs to reproduce?? and keep the process going year after year??  But as long as big business keeps getting their way the crab population will continue to decline untill they are an endangered species too.(hey I bet every landscaper out there wishes he could extend his season too,It's just SOME THINGS WERE NEVER MENT TO BE DONE YEAR ROUND in this part of the country, like growing vegetables, selling heating oil, and mowing the grass    and all the other businesses that are seasonal! But hey, keep squeezing the lemon,and sooner or later it will run out of juice      
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« Reply #6 on: December 13, 2003, 06:03:58 PM »

WHO IS WASTING MONEY ON THIS SRUVEY?Huh?
YOU BETTER BELIEVE WE ALL ARE PAYING FOR IT, ONE WAY OR ANOTHER!
OUR TAXES, FEES AND SUBSIDIES
TOO MANY BUREACRATS TRYING TO MAKE LEMONAIDE OUT OF THIS ONE LEMON

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Qcrab
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« Reply #7 on: December 13, 2003, 07:24:36 PM »

Crabguy

No thoughts on the "Fruit Cake" thing?

I heard they are either really tire bits or dates.

Happy Happy!

Qcrab
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« Reply #8 on: December 13, 2003, 09:29:40 PM »

 Huh Ah, wiseguy, this particular dredging, which was the subject of my post above, is not a commercial dredging operation.  It is a scientific endeavor aimed at determining the status of the blue crabs in the Chesapeake Bay.  Read the introduction portion again:

"The winter dredge survey is the only Bay-wide fishery-independent effort to assess status of the blue crab population in the Chesapeake Bay. The dredge survey produces estimates of abundance, indices of recruitment, indices of female spawning potential, and estimates of relative exploitation of the blue crab by commercial and recreational fisheries. A pilot version of the study was first conducted in 1988 with the cooperation of the University of Maryland Chesapeake Biological Lab (CBL). In 1989 the Virginia Institute of Marine Science (VIMS) joined the survey, and the two states continue to sample each winter from December through March."

The crabs which are dredged are released back into the bay just as soon as the pertinent data is recorded.  The crabs immediately head for the bottom and rebury themselves.  The findings from this annual winter dredge survey will benefit you, me and all the other crabbers that ply the waters of the Chesapeake Bay.  

Crabpop  
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waterboy
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« Reply #9 on: December 18, 2003, 08:38:21 PM »

I think we know why he's the wiseguy.
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Qcrab
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« Reply #10 on: December 18, 2003, 09:27:25 PM »

Who me?
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Qcrab
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« Reply #11 on: December 18, 2003, 09:38:21 PM »

AHHH I discovered BBQ CRABS today!

I put a dozen crabs in ice water today and reached in...VERY AFRAID!

They were asleep? Very little movement. I was still apprehensive. Broke off the claws, broke off the shell and cleaned em' out like they were steamed. Not as easy as when they are steamed but similar.

Separated the claws and the bodies, separated the two halves legs intact.

Dipped them in liquid smoke and water (1/2 and 1/2). Then sprinkled a mix of OLD BAY, cayenne, chili powder and smoked paprika. Let them sit for two hours. Fried them in peanut oil for about three minutes till they floated.

They were amazing! It was so different from what I was used to. They have the fried taste of softshells and the great taste of BBQ.

I still favor the steamed in OLDBAY and beer and vinegar.. but this was still great. Different but VERY good.

Next Im going try them in lime jello...heard it was awesome!
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