FINAL REPORT COVER SHEET
30 December 2010
Final Report to:
The Barnegat Bay Partnership
Project Title:
Assessing population structure, reproductive potential and
fishing effort for blue crabs in Barnegat Bay
Submitted by:
Paul R. Jivoff
Associate Professor
Department of Biology
Rider University
2083 Lawrenceville Rd
Lawrenceville, NJ 08648
TEL: 609-895-5421
FAX: 609-895-5782
e-mail: pjivoff@rider.edu
Background and Justification
Blue crabs are one of the most important commercial and recreational fisheries in New Jersey (Kennish
et al. 1984; Stehlik et al. 1998) and throughout the mid-Atlantic region (Jordan 1998). Over the past three
decades, blue crab populations in other mid-Atlantic estuaries (e.g., Chesapeake Bay and Delaware Bay) have
declined drastically (Abbe and Stagg 1996; Cole 1998; Uphoff 1998). These reductions may stem from a
number of factors including loss or degradation of habitat for recruits and juveniles (Lipcius et al. 2005),
reduced water quality (Mistiaen et al. 2003), and significant natural and fishing mortality (Lipcius and
Stockhausen 2002). Over the past decade in the mid-Atlantic region, as crab catches continue to decline in
Delaware Bay, the relative importance of New Jersey blue crab populations has increased 10-fold in terms of
both commercial landings and economic value (NOAA fisheries data). Some of this increase stems from New
Jersey estuaries other than Delaware Bay. For example, in the past decade, Barnegat Bay’s percentage of New
Jersey’s blue crab catch has quadrupled (NJDEP fisheries data; Figure 1). As the relative importance of blue
crab populations in estuaries like Barnegat Bay increases, the extent of fishing effort and the potential for user
conflicts may also increase. Therefore it is critical to gather information about the population status and the
extent of fishing effort (commercial and recreational) on blue crab populations in estuaries like Barnegat Bay.
Indeed, as indicated in the BBNEP Monitoring Program Plan (MPP): an assessment of the seasonal availability
and habitat use patterns associated with finfish and blue crab resources should be conducted for Barnegat Bay.
Objectives
The objectives of this study were to examine the relative abundance and population structure (e.g., size
structure, sex ratio) of adult blue crabs in Barnegat Bay using field sampling with traps and otter trawl. I also
measured temporal and spatial variation in aspects of the reproductive potential (e.g., sperm stores of both sexes,
brood production of field-caught females) and movement patterns of adult crabs (with mark-recapture data).
Mark-recapture data and direct counts of commercial crab traps (in selected locations) also provide information
on the temporal and spatial extent of fishing effort (commercial and recreational) on adult blue crabs in the Bay.
2
Methodology
Field Sampling: I examined adult blue crab population structure (abundance, size composition, sex
ratio) using baited traps sampled daily for four consecutive days, every other week from June-August and
sampling one day every other week in September. Sampling via otter trawl also occurred twice per month
(June-August) at the same sampling areas as trapping (see below), but not simultaneously with trapping, and
after at least seven consecutive days without trapping. Trawling occurred at a constant speed (2,500 rpm) for
two minutes using a 4.9 m otter trawl with 6 mm cod end mesh. Barnegat Bay was divided into seven
approximately equal sized areas (each ~8km long; see Figure 2) and four sampling sites in each area were
established using GPS. Each sampling day, four traps were randomly assigned to one of the four sites in each
area (and placed at least 50m apart from one another). Crabs were separated by trap or trawl haul, returned to
the Rutgers University Marine Field Station, and measured for carapace width, age, sex, sexual maturity, molt
stage, limb loss and regeneration, and ovigerous stage (adult females). Sexual maturity and molt stage were
determined using previously established methods (Jivoff 1997). Crabs from these collections were also used for
measurements of reproductive potential (see below). Physical characteristics near the first and last trap in each
site including depth, salinity, temperature, and dissolved oxygen were taken with a hand-held YSI datalogger.
In selected areas (e.g., those where commercial fishermen used individual surface buoys to mark each trap), on
each sampling day, I counted the number of commercial crab traps seen in-route from the current sampling site
to the next day’s site.
Reproductive Potential Studies: A weekly sample of crabs from five size categories of each sex from
each area were frozen for subsequent dissection and measurement of reproductive potential: sperm stores and
seminal fluid weight in males; sperm stores, ovarian weight and developmental stage, and brood stage in
females using previously established techniques (Jivoff 1997; Hines et al. 2003).
Mark-Recapture Studies: Each month, a sample of crabs of each sex was tagged and released from one
site in each area. The tags were plastic Floy tags affixed to the carapace using malleable stainless steel wire
wound around the lateral spines (Aguilar et al. 2005). Information imprinted on the labels included “BBNEP”,
contact phone number, individual tag number and requested the recording of the following data: tag number,
3
capture date, capture location (GIS coordinates if known), capture depth, and capture gear. Captors submitted
this information via phone or via my internet blog site (njbluecrabs.wordpress.com).
Results
Physical Characteristics
Temperature varied by month (F3,391=79.0, P<0.0001), decreasing from July through September,
however not by sampling area (F6,391=1.7, P=0.123) (Figure 3A). As expected, salinity varied by month
(F3,391=117.5, P<0.0001), peaking in July or August, and by sampling area (F6,391=334.9, P<0.0001), with a
gradient of decreasing salinity between areas 4 and 7 (Figure 3B). Dissolved oxygen varied by month
(F3,377=47.0, P<0.0001), with values dropping between June and July followed by a steady increase through
September, however not by sampling area (F6,377=0.15, P=0.999) (Figure 3C). Depth varied by both month
(F3,391=4.7, P=0.003), presumably due to sampling during different tides, and by sampling area (F6,391=10.7,
P<0.0001), primarily because areas 2 and 3 were relatively shallow but the absolute differences were 0.5m or
less (Figure 3D).
Adult Population Structure
Abundance
A total of 5,071 blue crabs were captured in traps over the course of this study; 3,668 adult males,
1,048 adult females, 109 juvenile males, and 245 pre-pubertal females. The predominance of males in our study
(3.5 times that of females) is consistent with that of the commercial blue crab fishery in Barnegat Bay during the
summer months which, since 2000, has caught between 3 and 8 times as many males as females (NJ DEP data).
I will report the abundance of crabs as catch per unit effort (CPUE) because the number of crabs caught was not
always based on four traps per site (e.g., due to missing or lost traps between sampling days). Trawl data were
excluded from this analysis because so few blue crabs were captured by trawling.
Overall, male CPUE was greater than that of females (F1,298=126.4, P<0.0001). Male CPUE exceeded
that of females at every sampling area except for area 4 (sex x area interaction; F6,298=10.7, P<0.0001) (Figure
4), the area closest to Barnegat Inlet which adult females use for spawning (see below). Male CPUE exceeded
that of females in each month except September (sex x month interaction; F3,298=13.2, P<0.0001). Because of
4
the prevalence of males, a more in-depth analysis of the temporal and spatial variation in the abundance of
males will follow. Male CPUE varied significantly among the months (F3,140=33.1, P<0.0001) and the sampling
areas (F6,140=12.0, P<0.0001). Except in areas 1 and 4, the abundance of males either peaked in July (areas 3
and 6) or was greater in July than at least two other months (areas 2, 5 and 7) (area x month interaction;
F18,140=2.6, P=0.001) (Figure 5). Among the locations, male abundance was largest at either sampling area 6 or
7 (i.e., at lower salinities) particularly as the summer progressed; in August the abundance of males in area 6
exceeded that of areas 1-4 and in area 7 the abundance of males was greater than that of areas 1-5 (Figure 5).
This spatial variation in the abundance of males was the driving force behind spatial variation in the sex ratio (#
of males : # of females) (see below).
Sex Ratio
The sex ratio varied both temporally (F3,274=7.3, P<0.0001), with September having the lowest M:F ratio
among the months, and spatially (F6,274=7.2, P<0.0001), with area 7 having the highest M:F ratio among the
sampling areas (Figure 6). The spatial variation in sex ratio exhibits the greatest change between areas 4 and 7
(see Figure 6), which also represents the largest change in salinity among the areas (see Figure 3B). There is a
significant negative relationship between salinity and M:F ratio between areas 4 and 7 (Y=-1.054X + 32.122)
with salinity explaining 71% of the variation in sex ratio. Female abundance does not vary significantly among
these sampling areas (F3,80=1.4, P=0.255) while male abundance does (F3,68=4.3, P=0.008) indicating sex ratio
variation, during the summer months, is regulated by the abundance of males and moderated by salinity.
Size
Male size varied significantly by sampling area (F6,3749=14.4, P<0.0001), with male size increasing until
areas 4 and 5 then falling in areas 6 and 7, and by month (F3,3749=51.4, P<0.0001), with male size increasing
until July or August then falling in September. The increase in male size between June and July or August was
exhibited at all sampling areas, except area 4, and the decrease in male size between July or August and
September occurred at areas 2, 3, and 5 (Figure 7).
The size of adult females alone did not vary significantly by sampling area (F6,847=1.4, P=0.202) but did
vary by month (F3,847=4.3, P=0.005) with July being larger than June. When both adult females and pre-pubertal
females were included in the analysis, female size varied temporally (F3,1266=39.0, P<0.0001) and spatially
5
(F6,1266=4.7, P<0.0001), with female size typically being large in July and September and small in June and
August (Figure 8). This pattern in female size was dependent upon the relative percentage of pre-pubertal
females in the population (i.e., bringing down the average female size) which was typically high in June and
August. Monthly female size decreased as the percentage of pre-pubertal females in the population increased
(Y=-34.76X + 135.18) and the percentage of pre-pubertal females explained 97% of the variation in monthly
female size. Similarly, female size among the sampling areas decreased as the percentage of pre-pubertal
females increased in the population (Y=-35.86X + 135.35) and the percentage of pre-pubertal females explained
80% of this spatial variation in female size.
Reproductive Potential
Female Brood Production
The largest numbers of ovigerous females were concentrated closest to the two inlets in Barnegat Bay;
Little Egg Inlet near Tuckerton and Barnegat Inlet near Waretown (Figure 9). The abundance of ovigerous
females and the developmental stages of their eggs varied temporally (Figure 10), indicating that the spawning
season began in May (there were already females with late stage broods captured in early June) and ended in
August (all of the ovigerous females in September were captured in the first week and showed signs of having
recently released their broods). Variation in the distribution of the egg developmental stages and the number of
ovigerous females suggests there may be two cohorts of females producing eggs at different times within the
estuary; one that begins producing broods in May and perhaps ends by mid-July, and the other that begins
producing broods in early-July and continues through August. Another possible explanation for the within- and
between-week variation in the distribution of egg developmental stages is that individual females produce
multiple broods of eggs throughout the summer.
Female Sperm Stores
The seminal receptacles (i.e., sperm storage organs) and ovaries from females of 6 size categories were
extracted and weighed separately. The weight of seminal receptacles from females showing signs of recent
mating (i.e., early stage ovaries) did not vary significantly by sampling area (F6,86=1.5, P=0.202) or female size
(F1,86=1.3, P=0.255) but did vary significantly by month (F3,86=11.4, P<0.0001), with weights in August being
greater than any other month (Figure 11). This temporal variation in female sperm storage may be associated
6
with the temporal availability of large males; the abundance of males typically peaks in July (see Figure 5) and
male size typically peaks in either July or August (see Figure 7).
Male Sperm Stores
The spermatophore and seminal fluid sections of the vas deferentia were extracted from males of 6 size
categories and weighed separately. The weight of the spermatophore (F5,769=23.1, P<0.0001) and seminal fluid
(F5,769=20.9, P<0.0001) components varied significantly by male size category, with both components getting
heavier with increasing male size class (Figure 12). Spermatophore weight increased significantly from male
size class 1 to 3 then again from size class 5 to 6. Seminal fluid weight increased significantly from male size
class 1 to 4. These results suggest that male reproductive potential increases with male size such that large
males have the capacity to provide females with greater amounts of both sperm and seminal fluid.
Movement
During the course of the study, 987 crabs were tagged and released and 99 were recaptured (10%
recapture rate), which is a comparable recapture rate with other tag-recapture studies performed on blue crabs
(e.g., Aguilar et al. 2005). Recaptures were evenly distributed between commercial (48% of recaptures) and
recreational fishermen (52% of recaptures). This differs from other blue crab tagging studies which typically
report the overwhelming majority of recaptures from commercial fishermen (e.g., Aguilar et al. 2005). There
was considerable variation in the distance traveled by tagged crabs but for at least the first 6 days, crabs tended
to travel farther the longer they were at large (Figure 13). The sampling areas are approximately 7km long and
most recaptured crabs remained in the same sampling area in which they were tagged however, some crossed
sampling area boundaries with the longest distance traveled being approximately 15 km by a crab at large for 15
days (Figure 12). Tagged crabs were recaptured relatively quickly; it took only 5 days to recapture 60% of the
tagged crabs (Figure 12). In other blue crab tagging studies, 20 days were required to recapture a similar
percentage of tagged crabs (Aguilar et al. 2005). The recapture results suggest that fishing in Barnegat Bay may
be relatively intense and that both commercial and recreational fishing represent important sources of fishing
mortality.
7
Summary and Conclusion
There is considerable temporal and spatial variation in various aspects of adult blue crab population
structure in Barnegat Bay including the abundance and size of both sexes and sex ratio. These aspects of
population structure influence the reproductive biology of blue crabs as seen in temporal and spatial patterns in
different measures of the reproductive potential of both sexes. For example, the availability of males, especially
large males with greater sperm stores, may influence female reproductive potential by regulating the supply of
sperm and seminal fluid females obtain for brood production.
8
3500000
40
3000000
35
30
25
2000000
20
1500000
15
Barnegat Bay %
Pounds Caught
2500000
Delaware
1000000
10
New Jersey
Barnegat Bay
500000
5
0
% Barnegat Bay
0
1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
Year
Figure 1. Annual summer-time (June-August) catch (in pounds) of blue crabs in Delaware, New Jersey, and
Barnegat Bay (left Y axis) and the percentage of the total New Jersey summer-time catch represented by
Barnegat Bay (right Y axis). Data from NOAA and NJDEP.
7
6
5
4
3
2
1
Figure 2. Map of Barnegat Bay showing locations
of seven equal sized sampling areas for this study.
9
A
Temperature (oC)
28
26
24
22
20
18
16
14
12
10
28
26
24
22
20
18
16
14
12
10
9
8
7
6
5
4
3
2
1
0
2.25
Month
6
7
8
9
1
2
3
4
5
6
7
Salinity (ppt)
B
Month
6
7
8
9
1
2
3
4
5
6
7
Dissolved Oxygen ( mg/L)
C
Month
6
7
8
9
1
2
3
4
5
6
7
Depth (m)
D
2.00
1.75
Month
6
7
8
9
1.50
1.25
1.00
1
Tuckerton
2
3
4
5
West Creek Manahawkin Waretown Forked River
6
7
Bayville
Cedar Grove
Sampling Area
Figure 3. Monthly (+ 1SE) physical characteristics of the sampling areas including temperature (A), salinity (B),
dissolved oxygen (C), and depth (D). Towns nearest to the sampling areas are included on the X axis.
10
10
9
Catch per Unit Effort
8
7
6
Males
Females
5
4
3
2
1
0
1
Tuckerton
2
3
4
5
West Creek Manahawkin Waretown Forked River
6
7
Bayville
Cedar Grove
Sampling Area
Figure 4. Catch per unit effort (+ 1SE) of males and females in each sampling area, June-September 1998.
Towns nearest to the sampling areas are included on the X axis.
16
14
Catch per Unit Effort
12
10
8
6
4
Month
6
7
8
9
2
0
1
Tuckerton
2
3
4
5
West Creek Manahawkin Waretown Forked River
6
7
Bayville
Cedar Grove
Sampling Area
Figure 5. Monthly (+ 1SE) catch per unit effort of males in each sampling area. Towns nearest
to the sampling areas are included on the X axis.
11
20
18
16
M:F Ratio
14
12
10
8
6
4
2
0
1
Tuckerton
2
3
4
5
West Creek Manahawkin Waretown Forked River
6
7
Bayville
Cedar Grove
Sampling Area
Figure 6. Male:Female ratio (+ 1SE) in each sampling area. Towns nearest
to the sampling areas are included on the X axis.
140
135
Size (mm)
130
125
120
Month
6
7
8
9
115
110
1
Tuckerton
2
3
4
5
West Creek Manahawkin Waretown Forked River
6
7
Bayville
Cedar Grove
Sampling Area
Figure 7. Monthly (+ 1SE) male size in each sampling area. Towns nearest
to the sampling areas are included on the X axis.
12
140
Size (mm)
130
120
110
Month
6
7
8
9
100
1
2
Tuckerton
3
4
5
West Creek Manahawkin Waretown Forked River
6
7
Bayville
Cedar Grove
Sampling Area
Figure 8. Monthly (+ 1SE) female size in each sampling area. Towns nearest
to the sampling areas are included on the X axis.
100%
103
200
66
305
113
130
95
Percent of Ovigerous Females
90%
80%
70%
60%
Non-ovigerous
Ovigerous
50%
40%
30%
20%
10%
0%
1
Tuckerton
2
3
4
5
6
West Creek Manahawkin Waretown Forked River Bayville
7
Cedar Grove
Sampling Area
Figure 9. The percentage of ovigerous and non-ovigerous females in each sampling area,
June-September 2008. Numbers inside bars represent sample size of all adult females
captured at the area. Towns nearest to the sampling areas are included on the X axis.
13
100
103
90
15
77
109
10
7
80
60
Egg Stage
4-late
50
40
3-mid-late
30
2-mid
20
1-early
10
0
1
June
2
3
4
July
Sampling Week
5 August 6
Figure 10. The weekly percentage of ovigerous females with different egg
developmental stages females in each sampling area. Numbers inside bars
represent sample size of ovigerous females. The months for each sampling
week are included on the X axis.
4.5
4.0
Seminal Receptacle Weight (g)
Percent
70
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
6
7
8
9
Month
Figure 11. Weight (+ 1SE) of seminal receptacles of females in early ovarian
development, June-September 2008.
14
2.0
1.5
1.0
Spermatophore
Seminal Fluid
0.5
0
100-109
110-119
130-139
140-149
120-129
Male Size Category (mm)
>150
Figure 12. Weight (+ 1SE) of the spermatophore and seminal fluid components in the
vas deferentia of males from 6 size categories, June-September 2008.
15.0
100
14.0
90
13.0
12.0
80
11.0
Cumulative Percentage
Distance Traveled
60
10.0
9.0
8.0
50
7.0
6.0
40
5.0
30
4.0
3.0
20
2.0
10
1.0
0
0.0
1
2
3
4
5
6
8
9
10
11
12
13
14
15
16
17
18
19
20 >20
Days at Large
Figure 13. The cumulative percentage and distance (+1 SD) traveled of recaptured crabs
during the number of days they were at large, June-September 2008.
15
Distance Traveled (km)
70
Cumulative Percentage
Weight of Component (g)
2.5
Bibliography
Abbe GR, Stagg C (1996) Trends in blue crab (Callinectes sapidus Rathbun) catches near Calvert Cliffs,
Maryland, from 1968 to 1995 and their relationship to the Maryland commercial fishery. Journal of
Shellfish Research 15: 751-758
Aguilar R, Hines AH, Wolcott TG, Wolcott DL, Kramer MA, Lipcius RN (2005) The timing and route of
movement and migration of post-copulatory female blue crabs, Callinectes sapidus Rathbun, from the
upper Chesapeake Bay. Journal of Experimental Marine Biology and Ecology 319: 117-128
Cole RW (1998) Changes in harvest patterns and assessment of possible long-term impacts on yield in the
Delaware commercial blue crab fishery. Journal of Shellfish Research 17: 469-474
Hines AH, Jivoff PR, Bushmann PJ, van Montfrans J, Reed SA, Wolcott DL, Wolcott TG (2003) Evidence for
sperm limitation in the blue crab, Callinectes sapidus. Bulletin of Marine Science 72: 287-310
Jivoff P (1997) Sexual competition among male blue crab, Callinectes sapidus. Biological Bulletin 193: 368-380
Jordan SJ (1998) The blue crab fisheries of North America: research, conservation, and management. Journal of
Shellfish Research 17: 367-587
Kennish MJ, Vouglitois JJ, Danila DJ, Lutz RA (1984) Shellfish. In: Kennish MJ, Lutz RA (eds) Ecology of
Barnegat Bay, New Jersey. Springer-Verlag, New York, pp 171-200
Lipcius RN, Seitz RD, Seebo MS, Colon-Carrion D (2005) Density, abundance and survival of the blue crab in
seagrass and unstructured salt marsh nurseries of Chesapeake Bay. Journal of Experimental Marine
Biology and Ecology 319: 69-80
Lipcius RN, Stockhausen WT (2002) Concurrent decline of the spawning stock, recruitment, larval abundance,
and size of the blue crab Callinectes sapidus in Chesapeake Bay. Marine Ecology-Progress Series 226:
45-61
Mistiaen JA, Strand IE, Lipton D (2003) Effects of environmental stress on blue crab (Callinectes sapidus)
harvests in Chesapeake Bay tributaries. Estuaries 26: 316-322
Stehlik LL, Scarlett PG, Dobarro J (1998) Status of the blue crab fisheries of New Jersey. Journal of Shellfish
Research 17: 475-485
Uphoff JH (1998) Stability of the blue crab stock in Maryland's portion of Chesapeake Bay. Journal of Shellfish
Research 17: 519-528
16