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Investigation of Octopus rubescens Population Dynamics:
Population Fluctuations at 200m Depth
in Monterey Bay
Kristin B. Hulvey
Advisors: Charles Baxter and James Watanabe
Hopkins Marine Station
Stanford University
Spring Quarter 1997
Permission is granted to Stanford University to use the citation and abstract of this paper.
Permanent address
Hopkins Marine Station
Stanford University
Pacific Grove, CA 93950
Abstract
In October of 1991, a National Marine Fishery Survey conducted at the 200 m
depth contour along a segment of sand-mud canyon bottom between
Ascension Canyon and Monterey Canyon in the Monterey Bay revealed an
unusually large population of Octopus rubescens (852 octopuses ha,
SE=179.8). When re-sampled in March of 1997 using a remotely operated
vehicle (ROV), the O. rubescens population was found to have significantly
decreased in size (3 octopuses ha", SE-3.2, p«0.001). Fishery effects on
predatory rockfish and factors affecting juvenile recruitment were found to be
the most plausible causes for the large 1991 O. rubescens populations. The
effect of these factors could not be established for the smaller 1997 populations
because of insufficient data. Large increases in numbers of harbor seals, Phoca
vitulina, within the bay may be contributing significantly to the O. rubescens
decline.
Introduction
The factors leading to population fluctuations in the deep sea benthic
communities of Monterey Bay are not clearly understood. Without a
knowledge of these factors, human impacts on marine ecosystems cannot
fully be quantified. In 1991, a National Marine Fisheries Survey discovered a
large population of Octopus rubescens (one every 0.8 m), along a section of
sand-mud canyon floor at 200 m depth (Charles Baxter, per. com.). Such an
unusual population provided an unique opportunity to investigate factors
which may be having obvious effects on a population sizes of a species in the
Monterey Bay.
O. rubescens is found from Alaska to Baja California to the Gulf of
California (Hochberg and Fields, 1980). The life span of the O. rubescens
ranges from one (Hochberg and Fields, 1980) to two years (Dorsey, 1976), with
juveniles spending an unknown amount of time in a pelagic phase. While
in the water column, juveniles stay at depths of 400 m or less (Hunt, 1996),
and feed mainly on zooplankton (Dorsey, 1976). Adult O. rubescens inhabit a
variety of habitats ranging from shallow water kelp beds to sand-mud canyon
bottoms at up to 250 m depth, and feed mainly on crustaceans, mollusks, and
fishes (Hochberg and Fields, 1980).
The goals of this research were to both determine how the population
of O. rubescens along the transect had changed from 1991 to 1997, as well as to
investigate some of the factors which may be causing changes in populations
of the O. rubescens.
Due to the history of commercial fishing in the area of the transect, as
well as the life history of the O. rubescens, fishery activity and factors affecting
O. rubescens survival during their time in the pelagic phase were both
investigated as possible causes of population fluctuation. Additionally,
because of increases in numbers of harbor seals in the area, predation pressure
from seals was also investigated.
Methods
Study Site
The study site is located on a section of sand-mud ocean bottom at 200 m
depth between Monterey and Ascension canyons (Fig. 1). For a detailed
description of the transect area see Adams et al. (1995). The transect began at
N 36° 47’ latitude W 122° 06' longitude and extended for approximately
10 km, ending at approximately N 36° 48' latitude W 122° 08' longitude.
Transect Sampling
The 1991 transect was re-sampled on March 21, 1997 using the Monterey Bay
Aquarium Research Institute’s (MBARI) remotely operated vehicle (ROV),
Ventana. ROV location was measured every few seconds using a combined
navigational system consisting of a differential global positioning system and
a sonar system. RÖV depth and altitude off substrate were measured
simultaneously with location and were recorded to the nearest meter. Using
telemetry as a guide, the path of the ROV tracked the 200 m contour as closely
as possible. The average height above the substrate was 0.87 m, and average
speed of the ROV varied because of strong tail currents. For detailed
descriptions of the ROV camera specifications see Adams et al., (1995
Video Analysis
Video tapes of both the 1991 and 1997 transects were viewed at least three
times to identify organisms found along the transect. All visible organisms
on the video tape were identified to the lowest taxonomic level, and their
abundances were recorded.
O. rubescens were sub-sampled in order to quantify the large change in
population numbers observed from 1991 to 1997. Rockfishes were similarly
sub-sampled due to possible biological interactions between octopus and
rockfish populations. To obtain sub-sampled areas, the 1991 and 1997
transects were divided into 52 and 93 segments, respectively, with each
segment being as close to 100 m as possible. The length of each segment was
determined from the difference between successive location estimates. Ten
segments were randomly chosen from each transect sample, and octopus and
rockfish numbers were counted and recorded.
Total transect area was calculated by multiplying transect width by
transect length. Transect length was determined by summing distances
between successive Northing and Easting measurements. A total of 5984.1 m
of 1991 transect and 9154.1 m of 1997 transect was used in the analysis. The
width of the 1991 transect was 1.8 m (Adams et al., 1995). The width of the
1997 transect, 3.1 m, was calculated by using the constant distance between two
laser points (28 cm) and extrapolating the transect width from the total
monitor width. Because the laser system was only operating at the end of the
transect run, the width calculated from the lasers during this time was
assumed to be the same as the average width of the whole transect. 1o
account for differing total areas seen in 1991 and 1997, all density estimates
were normalized to number of organism per hectare (10,000 m2).
Portions of the video tape from 1991 and 1997 were unusable and
omitted from transect analysis. Reasons for omission included: the ROV was
too far above the substrate for organisms to be counted and identified, the
ROV was sitting on the bottom motionless, the ROV wandered + 20 m off of
the 200 m contour, or the recorded length of a segment between two time
points was too large to be reasonable.
A G-test of independence (Sokal and Rohlf, 1981) was used to test for
changes in relative abundance occurring from 1991 to 1997. Additionally the
Shannon Index was used as a measure of species diversity (-2p, In(p), where
p=proportion of the i“ species; Magurran, 1988). Student's t-tests were
performed on sub-sampled data in order to establish if octopus and rockfish
numbers had changed significantly between 1991 and 1997. Octopus variances
were significantly different (F-test, P20.01), requiring the use of an unequal
variance t-test. Rockfish data were log transformed resulting in homogenous
variances (F-test, P-0.28), and t-tests were performed on these transformed
data.
Fishery Data
Fishery data from 1980 to 1986 were obtained from the California Department
of Fish and Game. To monitor fish landings, the Department of Fish & Game
has divided the Monterey Bay into a series of 18 km by 18 km quadrats.
Fishing activity is monitored in each quadrat, and type of fishing, depth of
fishing (if trawling is used), length of fishing run, and pounds of each species
or market group caught are recorded. For this study, data of rockfish taken by
otter trawl along the 200 m + 20 m depth contour from Fish & Game block 518
were used. Most of the transect fell within this quadrat area, although the 200
m + 20 m depth contour within block 518 was about twice as long as the total
transect length.
Pounds of rockfish taken per year (hereafter referred to as "take") were
used as an indicator of fishing stress on the rockfish population, while
pounds of rockfish per hour of fishing time was used as a rough indicator of
rockfish abundance. It was assumed that changes in take and abundance
along the whole 200 m + 20 m depth contour in block 518 fluctuated in the
same manner as take and abundance along the length of the transect.
Juvenile Octopus Data
Numbers of juveniles in the water column were calculated from mid-water
transect videos obtained from MBARI's video database. All mid-water
transects occurred at a regularly sampled site at the axis of the Monterey
Canyon, and spanned the years of 1989 to 1995. A search of the general
database, which consisted of video through 1994, resulted in 40 transects
where octopuses were recorded. A second database consisting mainly of 1995
video was searched, and an additional 9 transects were identified as having
octopuses. Sections of video annotated as having sightings of single
octopuses were not examined, and a single octopus was recorded to have been
seen. Sections of video annotated as having sightings of octopus populations
were viewed and the number of octopuses seen were recorded.
Quantifying numbers of juveniles became much more difficult if
swarms of octopus were spotted. Swarms of octopuses often faded into the
distance, with individuals fading in and out of focus. Additionally, because
swarms often covered an area larger than the camera’s field of view,
individuals swimming off screen could swim back through a moment later
and be counted more than once. In order to standardize how groups of
octopuses were counted, a grading system was employed. A population score
of 5 was given to any swarm where 5 to 9 individuals could be seen in the
video frame at any given moment. A population score of 10 was given for to
swarms of 10 to 14 individuals, and a score of 15 was given to swarms of 15 or
greater. For groups of less than 5 individuals, the exact number present was
determined. No swarms greater than 20 octopuses per frame were observed.
Total numbers of juveniles seen per year were divided by total
numbers of dive hours for that year. Dive hours were estimated to the
nearest half an hour and included videos where no octopuses were seen. The
resulting index was a measure of juvenile octopus abundance per year.
Chlorophyll Data
Mean chlorophyll levels from 1989 to 1996 were used as an indicator of mean
phytoplankton abundance in the upper portion of the water column.
Average yearly measurements of chlorophyll levels (ug 1*) occurring at 0
depth in the water column were calculated using chlorophyll measurements
obtained from MBARI (Chavez, unpubl. data). It was assumed that levels of
chlorophyll, and thus phytoplankton abundance at the surface, reflected
general food availability for juvenile octopuses.
From these data, regression relationships between chlorophyll and
juvenile octopus abundances were estimated using time lags of zero, one, and
two years. This relationship was used to establish relationships between
fluctuations in standing crop of phytoplankton, and thus possibly
zooplankton abundance, and fluctuations in juvenile octopus numbers.
Harbor Seal Data
Predation intensity by harbor seals was estimated from the occurrence of
octopus beaks in seal scat. To estimate numbers of beaks in scats from the
early 1990's, numbers of beaks found in seal scat taken from Elkhorn Slough
from January 1991 to December 1991 (Oxman, pers. com.) were averaged with
numbers of beaks found in Slough scats from May 1991 to May 1992 (Trumble,
1995). Estimates of numbers of beaks found in scats for the mid to late 1990's
was determined from scat samples collected from September 1995 to
September 1996 (Tomo Eguchi, unpubl. data).
The 1995-1996 beak numbers were determined by counting beaks
already extracted from scat samples obtained for a different study (Tomo
Eguchi, unpubl data). The greatest number of either upper or lower beaks was
used as the total number of beaks per scat. For both the early 90's and mid to
late 90's, total beak numbers found in scat samples over the year were divided
by total numbers of scats collected during the year to obtain an index of
average beaks per scat per year.
Results
Transect Analysis
Octopus rubescens decreased significantly from a mean of 852 ha“ in 1991 to 3
hal in 1997 (t-test, P«0.001, SE=179.8, 3.2, Fig. 2). An increase in body size was
also noted among octopuses from the two samples, from approximately 5 cm
in 1991 to 20 cm in 1997. Sizes were estimated as general size of the animal
when sitting on the substrate and not strictly mantle size.
Rockfish numbers increased significantly from a mean of 42 ha“ in
1991 to 54 ha“ in 1997 (t-test, P«.04, SE’s-32.3, 14.6, Fig. 3). Qualitative changes
were also noted for body size of rockfishes. Most rockfishes along the transect
in 1991 were approximately 20 to 40 cm total body length. The majority of
rockfishes observed in 1997 were also in this 20 to 40 cm size class. However,
there was an additional class of smaller rockfish, approximately 10 cm in
length or smaller. Distribution of the rockfish population also changed from
patchy in 1991, to more evenly spread along the transect in 1997.
Relative abundances of other species found along the transect changed
significantly (G.g=65.3, P«0.001). Octopus and rockfish populations were
excluded from this calculation because their significance was already
determined. Largest decreases in abundance from 1991 to 1997 included: the
sea urchin Allocentrotus fragilis, slender sole (Eopsetta exilis), sand star
(Luidia foliolata), and red sea star (Mediaster aequalis). Largest increases in
abundances from 1991 to 1997 included: bigfin eelpout (Lycodes cortezianus),
the sea star Rathbunaster californicus, and spotted cusk-eel (Chilara taylori)
(Fig. 4).
Species richness decreased from 27 in 1991 to 22 in 1997 and total
diversity, as measured by the Shannon index, decreased significantly (H' for
1991 = 1.53, H' for 1997 = 1.02,t-7.41, P««0.001, Table 1)
Fishery Data
Rockfish take and abundance fluctuated from 1980 to 1996 between years of
high take-high abundance and low take-low abundance (Fig. 5). Take is
assumed to be a measure of stress inflicted on the rockfish population by the
fishery because high take years are followed by low abundance years. It was
assumed, therefore, that a high take could negatively affect the rockfish
population by reducing it to a low level in subsequent years. A large take of
approximately 45000 lb. of rockfish in 1990 was followed by a low abundance
of 250 lb. of rockfish caught fishing hr’ in 1991. In 1995 and 1996 a take of
approximately 25,000 lb. and 48,000 lb. of rockfish was coupled with
abundances of approximately 300 and 400 lb. fishing hr". The large take in
1996 reflects a larger number of hours of fishing rather then a high abundance
of rockfish. From the previously seen pattern of high take years being
followed by low abundance years, a low abundance of rockfish is expected in
1997.
Juvenile Octopus Data
There was a general trend of decreasing numbers of juvenile octopuses in the
water column from 1989 (=2.25 octopus hr*) to 1993 (=0.4 octopus hr*), with
slight increases in numbers occurring in 1994 (20.41 octopus hr*) and 1995
(=0.08 octopus hr, Fig. 6).
Chlorophyll abundance versus juvenile octopus abundance were
plotted in three different ways. While all scatter plots had positive slopes,
none had statistically significant correlations. This lack of statistical
significance was probably due to the small number of points plotted as
opposed to a lack of correlation, especially for the octopus abundance which
was lagged by one year. For directly plotted abundances, the best fit line
indicated that fluctuations in mean yearly chlorophyll accounted for only 1%
of the fluctuations seen in octopus numbers (Fig. 7). For abundances plotted
lagging octopus abundance one and two years behind chlorophyll abundance,
resulting chlorophyll fluctuations explained 34% and 13% of the fluctuations
seen in juvenile octopus abundance (Fig. 8 and 9).
Harbor Seal Data
The numbers of octopus beaks per scat sample decreased from approximately
5.5 during the early 1990’s to approximately 1.5 during the mid to late 1990's
(n=547, 99, Fig. 10).
Discussion
Evidence suggests that fishing pressure, factors affecting the numbers of
pelagic juvenile octopus recruiting to the benthic community, and increases
in predation pressure by increasing numbers of harbor seals contributed to the
significant fluctuations in Octopus rubescens populations between 1991 and
1997. Additionally, these or other factors may be affecting community-wide
patterns of relative abundance and diversity.
The study site is located in one of three most heavily fished areas in the
Monterey Bay, with the 200m depth contour being the most heavily trawled
depths in this site (Bob Leos, Dept. of F&G, per. com.). Rockfishes, a target
species for trawl fisheries, use cephalopods as a major food source (Houk,
1992). Increases in rockfish numbers, from 46 ha“ in 1991 to 54 ha in 1997,
accompanied by decreases in octopus numbers, from 853 ha’ to 3 ha", support
the idea of a significant interaction between these two populations. Because
of this, fishery activity along the transect area may be influencing octopus
populations by affecting the numbers of predatory rockfish.
It is not possible to determine the exact relationship between octopus
abundance and fishery pressure. However, based on the assumption that
rockfish take is affecting rockfish abundance and that rockfish abundance is
affecting octopus abundance, the large rockfish take in 1990 followed by lower
rockfish abundance in 1991 may have released octopuses from predation
pressure in 1991. Such a release may have allowed octopus survival rates to
be greater than normal along the transect, resulting in the larger population
of octopuses found in 1991.
It is harder to establish how fishing activity may have contributed to
the greatly reduced octopus numbers seen in 1997. For reasons presented
above (see results), fishery data predict a low rockfish abundance in 1997.
Findings from video analysis, however, indicate that rockfish populations
have increased significantly over the area of the transect in 1997. Although
this result seems to contradict fishery predictions, it may be that the 1997
rockfish population, although decreasing from populations found in 1996, is
still larger than populations found in 1991. Regardless, it is unlikely that the
small increase in rockfish abundance, from 46 to 54 ha", fully explains the
large decrease in octopus population found in 1997. More research is needed
to establish connections between fishery activity and octopus population
decreases.
13
A second factor that may have led to the large change in octopus
numbers along the transect is a fluctuation in the numbers of juveniles
recruiting to the benthic community. This hypothesis is supported by high
abundances of octopuses in the two years immediately preceding the large
population of benthic octopuses found in 1991 (Fig. 6). Because large
abundances of juveniles were found in the two years preceding 1991, the data
are consistent for both a one and two year O. rubescens life cycle.
It is difficult to establish how recruitment class size affected 1997
benthic numbers because class size numbers for 1996 are not available.
Juvenile abundance since 1989 generally decreased (Fig. 6), with small
increases in both 1994 and 1995. If O. rubescens has a two year life cycle, the
1995 class may indicate the 1997 population size. Although juvenile
abundance in 1995 was lower than in 1989 and 1990 (20.8 juveniles hour
compared to =1.8 to 2.3 juveniles hour’), it is unlikely that this slight decrease
in abundance alone accounts for a change of two orders of magnitude in
benthic octopus numbers from 1991 to 1997.
Potential factors that may affect juvenile abundance include: the size of
the larval pool produced by benthic adults, the availability of food for
juveniles in the water column, or sources of juvenile mortality such as
predation. Since juvenile abundance in the years after the large benthic
population of 1991 was smaller than previous years (Fig. 6), it seems unlikely
that adult abundance strongly influences juvenile abundance.
The juvenile's food chain is short: O. rubescens eats zooplankton,
which grazes on phytoplankton. Such a relationship makes it feasible that
fluctuations in the production of phytoplankton affect juvenile abundances
by affecting abundances of the juvenile’s prey. Juveniles feed upon mystids,
copepods and ostracods (Adams et al., 1995). Although there were no data
available on population sizes of these prey items during this study, the
positive slopes between juvenile octopus abundances and chlorophyll
concentrations suggests that food may indeed be important. The greater
correlation found between octopus abundances and chlorophyll
concentrations the previous year may be a result of delayed effects of
phytoplankton abundances on zooplankton abundances, and consequently
delayed effects on juvenile numbers. Factors that could affect prey abundance
include natural nutrient cycles in the Monterey Bay, changes in current
patterns or other meso-scale oceanographic processes. Because none of the
correlations were statistically significant, more data points are needed
positively determine the relationship between chlorophyll concentration and
juvenile octopus abundance.
Sources of predation on juvenile octopuses are not known, but
mortality rate among juveniles is probably high. The lower r’ for the
correlation of octopus abundances and chlorophyll concentrations from two
years prior may be due to a greater chance for sources of mortality, including
predation while in the water column, to occur between the recruitment of
one cohort and the production of the next generation of juveniles. More
15
research is required to discover what effects predation may have on
abundances of juvenile octopuses in the water column.
The final factor that may have led to population fluctuations among O.
rubescens along the transect area is the increase in predation pressure from
harbor seals. With exception of the El Nino years of 1983 and 1993, California
harbor seal populations have been growing every year since the Marine
Mammal Protection Act was passed in 1972 (Barlow et al., 1995). An annual
growth rate between the years of 1982 and 1994 was calculated as 4.1% (Barlow
et al., 1995). However, annual growth rates have been estimated at as high as
14.7% (Oxman, 1995
Harbor seals are benthic predators that consume 2.5-5.0% of their body
weight per day (Oxman, 1995). Through such feeding habits, harbor seals may
affect populations in the Monterey Bay. Oxman (1995) found that octopus, as
measured by octopus beaks in scat, accounted for approximately 35% of prey
items found in Elkhorn Slough harbor seal scat during three seasons of the
year. Additionally, for these three seasons, octopus was the most common
prey item in the scat (Oxman, 1995). Seals forage over the entire depth
distribution range of O. rubescens: O. rubescens is found to 200 or 250 m,
while seals dive to 500 m (Tomo Eguchi, per. com.).
Because harbor seals are opportunistic predators, it is unlikely that the
decreases in numbers of beaks found per scat, from 5.5 in the early 1990 to 1.5
in the mid to late 1990's, were a result of a change in prey preference. More
likely such decreases in numbers of octopus beaks are reflections of shrinking
numbers of octopuses available as prey items.
It is difficult to extrapolate the Elkhorn Slough seal scat data to the area
of the 1991 and 1997 transects. The Elkhorn Slough seals tend to dive in areas
further to the south-east of the transect site, diving in and around the
Monterey Canyon closer to Elkhorn Slough (Tomo Eguchi, per com.).
Additionally, the majority of dives, 11329 out of 16403 were to depths less
than 75m (Tomo Eguchi, unpubl. data). In order to document the impact of
seal predation on the transect area, more research is required.
Along with the changes in octopus populations along the transect from
1991 to 1997, changes in the rest of the benthic community were also evident.
Results from the G-test of independence suggest that relative abundances of
species present on the transect have changed significantly over this time
period (Table 1, Fig. 4). Additionally, Shannon's Diversity Index indicates a
significant decrease in diversity from 1991 to 1997. From the research
conducted, it is difficult to conclude what these changes may indicate;
however, as seen through the investigation of fluctuations in O. rubescens
populations, many factors may be working together to create the community
wide changes found along the transect.
Conclusion
O. rubescens numbers on a transect along the 200 m depth contour in
Monterey Bay fluctuated significantly from 1991 to 1997. This study found
that the large population of O. rubescens seen along the transect in 1991 may
have been partially due to a combination of two factors. The first is lower
rockfish predation pressure on octopus populations in 1991 as a result of a
large rockfish take in 1990. The second is large 1989 and 1990 juvenile octopus
abundances in the water column recruiting to the benthic community in
1991.
It was harder to establish what factors may have contributed to smaller
populations of O. rubescens numbers in 1997 because of incomplete fishery
and juvenile octopus data. However, possible factors include greater
predation pressure from increased numbers of rockfishes and harbor seals.
Although not specifically investigated in this study, whole transect
changes such as increases in relative abundances and species diversity were
also observed. While this study has begun to investigate factors contributing
to changes in a population of O. rubescens at 200 m, further research is
required to elucidate the complex factors contributing to changes in deep sea
benthic communities in the Monterey Bay.
Acknowledgments
Special thanks to Chuck Baxter for providing the inspiration for this work,
and Jim Watanabe for helping to fit all of the pieces together, as well as for
helping with the nuts and bolts of the paper. Thanks to the many people at
MBARI including: Jim Barry and Patrick Whaling for help with ROV
coordination, Bruce Robison for juvenile octopus data, Francisco Chavez for
water chlorophyll data, and especially Nancy Jacobson for all her help with
data base searches and video lab research. Additionally, thanks to the many
others who provided information including: Bob Leos and Brenda Erwin at
the California Department of Fish and Game, Mary Yoklavich and Richard
Parish at National Marine Fisheries, Karen Light at the Monterey Bay
Aquarium, and Dion Oxman, Tomo Eguchi and Teri Nicholson of Moss
Landing Marine Laboratories for harbor seal data.
Literature Cited
Adams, Peter B. John L. Butler, Charles H. Baxter, Thomas E. Laidig,
Katherine A. Sahlin, and W. Waldo Wakefield. 1995. Population
estimates of Pacific coast groundfishes from video transects and swept-area
trawls. Fishery Bulletin. 93:446-455 (1995)
Barlow, Jay, Robert L. Brownell, Jr., Douglas P. DeMaster, Karin A. Fomey
Mark S. Lowry, Steven Ösmek, Timothy J. Ragen, Randall R. Reeves, and
Robert J. Small. 1995. Harbor seal (Phoca vitulina richardsi): California
stock. Technical Memorandum. NOAA fisheries/NMFS Southwest
Region.
Dorsey, E.M. 1976. Natural history and social behavior of Octopus rubescens
Berry. M.S. thesis, University of Washington.
Hochberg, F.G., Jr. and W.G. Fields. 1980. Cephalopoda. In: Intertidal
invertebrates of California. Morris, R.H., D.P. Abbott, and E.C. Haderlie
(eds.). Stanford University Press, Stanford. pp. 429-444.
Houk, James L. 1992. Rockfishes: Overview. In: California’s living marine
resources and their utilization. Leet, William S., Christopher M. Dewees,
and Charles W. Haugen (eds.). California Sea Grant, Davis. pp. 257.
Hunt, James C. 1996. The Behavior and ecology of midwater cephalopods
from Monterey Bay: submersible and laboratory observations. M.S. thesis,
University of California Los Angeles.
Magurran, Anne E. 1988. Ecological diversity and its measurement.
Princeton University Press, Princeton. pp. 34-39.
Oxman, Dion S. 1995. Seasonal abundance, movements, and food habits of
harbor seals (Phoca vitulina richardsi) in Elkhorn Slough, California. M.S.
thesis, Moss Landing Marine Laboratories. pp. 125.
Sokal, Robert R. and R. James Rohlf. 1981. Biometry, 2nd edition. W. H.
Freeman and Company, New York. pp. 691-778.
Trumble, Stephen J. 1995. Abundance, movements, dive behavior, hood
habits, and mother-pup interactions of harbor seals (Phoca vitulina
richardisi) near Monterey Bay, California. M.S. thesis, Moss Landing
Marine Laboratories.
Table 1. Number and Type of Organisms Found Along the Transect in 1991 and 1997
Common Name
t ha
Latin Name
ha
1991
1997
RED OCTOPUS
RedOctopus
Octopus rubescens
531
ROCKFISHES
Stripetail rockfish
Sebastes saxicola
Small Stripetail
Sebastes saxicola
Widow rockfish
Sebastes entomela
splitnose rockfish
Sebastes diploproa
Unknown spp.
SEA CUCUMBER
Parastichopsis
Seacucumber
DOVER SOLE
Doversole
Microstomus pacificus
REX/ENGLISH SOLE
Rexsole
Errex zachirus
Pleuronectes vetulus
English sole
SLENDER SOLE
Slendersole
Eopsetta exilis
SANDDAB
Pacific sanddab
Citharichthys stigmaeus
Unknown spp.
SKATES
Raja rhina
Longnoseskate
California skate
Raja inornata
Unknown spp.
EELPOUT
Lycodes cortezianus
Bigfin eelpout
Unknown spp.
SUNFLOWER STAR
Rathbunaster californicus
Rathbunaster
SAND STAR
Sand star
Luidia foliolata
ASSORTED ECHINODERMS
Red sea star
Mediaster aequalis
Feather star
Florometra serratissima
Gorgonocephalus eucnemis
Basket star
URCHINS
Allocentrotus fragilis
INVERTEBRATES I
Tannercrab
Lopholithodes foraminatus
Box crab
Chionoecetes bairdi
Loligo opalescens
Squid
Pandalus platyceros
Spot prawn
INVERTEBRATES II
Seaslug
Pleruobranchaea californica
Ptilosarcus gurneyi
Sea pen
White plume anenome
Metridium giganteum
Spongespp.
ROUNDFISH
Pacific argentine
Argentina sialis
Clupeidae
Herring
Sablefish
Anoplopoma fimbriam
Lingeod
Ophiodon elongatus
Unknown spp.
BOTTOM FISH
Spotted ratfish
Hydrolagus colliei
Poachers
Agnidae
EEL-LIKE FISH
Pacific Hagfish
Eptatretus stoutii
Spotted cusk-eel
Chilara taylori
Total Number of Organisms
1253
Total Number of Species
Although no individuals from these categories were recorded when area was normalized to 1 ha,
individuals were seen on the transect and therefore included in total number of species seen.
861
Figure Legend
Figure 1: Study site in Monterey Bay with transect area marked by solid dark
line along the 200 m depth contour.
Figure 2: Decrease in O. rubescens population along the transect site from
1991 to 1997 with area normalized to one hectare. Vertical bars
indicate standard error. Numbers used reflect abundances found
along transect sub-samples.
Figure 3: Inverse population changes of O. rubescens and rockfishes along the
transect from 1991 to 1997. O. rubescens decrease and corresponding
rockfish increase. Numbers used reflect abundances found
along transect sub-samples.
Figure 4: Species population fluctuations along the transect from 1991 to 199
Figure 5: Fishery impact on the rockfish population from 1980 to 1996. Plot of
pounds of rockfish taken per year and rockfish abundance per year
(Abundance is determined by pounds of rockfish taken per hour of
fishing).
Figure 6: Fluctuation in abundances of juvenile octopuses in the water
column from 1989 to 1995.
Figure 7
Correlation between juvenile O. rubescens abundance and
chlorophyll concentrations, data from 1989 to 1995
Juvenile Octopus Abundance"= 0.686 (+ 1.500) + 0.104 (+ 0.455)
[chlj, r°= 0.01. Correlation is not significant, P-0.83.
Correlation between juvenile O. rubescens abundance and
Figure 8
chlorophyll concentrations. Octopus abundances are lagged one
year behind chlorophyll concentrations. Data collected from 1989 to
1996. "Juvenile Octopus Abundance"= -0.680 (+ 1.061) + 0.448 (+
0.311) (chl.],r’= 0.34. Correlation is not significant, P-0.22.
Figure 9:
Correlation between juvenile O. rubescens abundance and
chlorophyll concentrations. Octopus abundances are lagged two
year behind chlorophyll concentrations. Data collected from 1989 to
1996. "Juvenile Octopus Abundance"=-0.184 (+ 684) + 0.137 (+ 0.208)
[chl.],r°= 0.13. Correlation is not significant, P-0.56.
Figure 10: Average number of octopus beaks found in 1991 and 1996 harbor
seal scat samples. Sample sizes = 547 and 99.
Figure 1


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