An Analysis of Trap Saturation and the Behavioral Basis of Catchability

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Project Type: 
Research
Project Number: 
R/FMD-148
Inception Date: 
1997
Completion Date: 
1998
Theme Area: 
Fisheries Resources

Participants:

Winsor Watson UNH - Department of Biological Sciences Principal Investigator
W. Huntting Howell UNH - Department of Biological Sciences Co-Principal Investigator

Students Involved:

Jenna Wanat UNH - Department of Biological Sciences
Steven Jury Bates College
Dave Kooritis UNH - Department of Biological Sciences
Glenn Brown UNH - Department of Biological Sciences
Abstract: 

This is a one year pilot study designed to:

1) Test the feasibility of using two new methods for estimating lobster abundance

2) Generate a number of testable hypotheses concerning the catchability of lobsters

3) Quantify saturation of lobster traps and the behaviors that cause it.

All three issues are directly related to catchability, which is the likelihood that a lobster will enter a trap and be captured. The applied value of understanding catchability can hardly be overstated. Lobster managers must have a thorough understanding of catchability to accurately assess lobster abundance and trends in the fishery. Harvesters are also interested in catchability because it has a direct influence on their catch, fishing efficiency and profits.

Given its importance to all aspects of the fishery, it is surprising that we still know very little about catchability, especially the behavioral mechanisms that influence it. If we had a better understanding of the behavioral factors that influence catchability, it would be possible to reduce the error associated with our present system of estimating lobster populations based on trap data.

Objectives: 

1) To develop trap saturation curves, with the assistance of local lobstermen, in areas with low, medium and high density of lobsters

2) To develop finer scale, 24-hour trap saturation curves in the same low, medium and high density study areas, using our recently developed Lobster Trap Video (LTV) system

3) To calculate a time to saturation (TTS) index using the aforementioned saturation curves and determine if TTS varies as a function of lobster abundance

4) To measure lobster approaches to a trap using LTV, and determine the relationship between approaches and actual lobster density (as determined by SCUBA survey)

5) To develop one or more testable hypotheses concerning the behavioral mechanisms that give rise to trap saturation, using sea sampling data and examination of the video data obtained with LTV

Methodology: 

Deploy standard traps in cooperation with local lobstermen in areas of three different densities at varying soak times to determine saturation curves. Deploy our lobster trap video (LTV) system on several occasions in these same areas to continuously record behaviors in and around the trap that influence saturation and catchability. Determine abundance in each area using standard traps, LTV and SCUBA surveys.

Rationale: 

Present estimates of lobster populations, necessary for management, are highly biased due to unknown factors that influence trap selectivity. An understanding of these behavioral mechanisms affecting catchability and trap saturation is necessary to reduce this error.

Results will be useful to: fisheries managers who can more accurately assess the relationship between catch and abundance, lobster biologists interested in behavior and ecology, and the lobster industry because the results may suggest ways to optimize existing fishing gear more cost effectively.

Accomplishments: 
Objective 1. Developing saturation curves.
 
We successfully completed this objective, although almost all the areas we fished along the N.H. coast had medium to high densities of lobsters so it was difficult to obtain the range of data we desired.
 
Objective 2: Saturation curves based on video data.
 
We also completed this objective and data obtained from LTV supported data collected with traditional traps. It is clear from the data obtained from LTV, and those obtained from traditional traps, that traps "saturate" in less than 24 hours, and in many cases in12 hours or less. However, it should be emphasized that our data is based on all lobsters, not just legal lobsters. It is likely, due to their low density, that the catch of legal lobsters does not saturate so fast. Further examination of our data, with this question in mind, is necessary to address this question.
Objective 3. Determine if ITS varies with lobster density.
 
As stated above, we were unable to find areas along the N.H. coast that differed significantly in density. We were able to obtain curves from high and low areas, but not over a range of densities. Nevertheless, based on the data collected from these sites it does not appear as if TTS is a good measure of density for two reasons. First, the variation is high, which makes it difficult to calculate the precise time when saturation is achieved. Second, saturation seems to take place within the first 12 hours. Perhaps a more appropriate index of abundance would be the total catch in the first 3 hours, which seems to vary with density, or the rate of saturation. Additional studies are necessary to determine what aspect of the trap saturation curve would be the best indicator of abundance.
 
While conducting surveys to find appropriate study sites we discovered that on the sandy substrate off the Wallis Sands beach in Rye, N.H., the density of lobsters fluctuates considerably during the summer. Therefore, we decided to use this site to examine the relationship between density and TTS in the future and this became an objective in a subsequent Sea Grant Proposal (which was successful). This advantage of this new study site is that the density varies considerably from June through November and the lobsters are easy to count because there are not places to hide.
Objective 4. Measure lobster approaches and determine if approaches vary with density.
 
We succeeded in accomplishing this objective, at least in part. We were able to use our LTV system to measure the number of approaches to a lobster trap. However, to our surprise the numbers were extremely high. We discovered that hundreds of lobsters appeared in our field of view every hour. It is obvious that in many cases the same lobsters were moving away from the trap, and then approaching it again. We estimate, based on observations of "identifiable" lobsters (missing claws etc.) that lobsters approach traps 10-15 times before entering. However, this varies with size. Larger lobsters are more likely to enter right away. Therefore, although we counted hundreds of different approaches, the number of different lobsters approaching per hour is unknown. In fact, due to this problem, we have come to the conclusion that it is not a good idea to use approaches as an index of abundance.
Currently, we are using several different techniques to determine, on average, how many times an individual lobster approaches a trap. This will allow us to convert all our observations to an estimate of the number of individual lobsters that approached.
 
In our subsequent grant we focused our attention on entries into the trap, rather than approaches, because they are easier to quantify, easier to see at night, and generally the same lobster does not enter and exit many times during a given soak.
Objective 5. Develop testable hypotheses concerning behavioral mechanisms underlying saturation.
 
We made four significant observations from our videos that enabled us to develop 2 testable hypotheses. First, a very large number of lobsters flow through a lobster trap during a given soak and >90% of them escape. Second, most lobsters escape through the entrance of the trap. Third, there is intense competition outside the entrance to the kitchen. Lobsters defend this area and look for an opportunity to enter the trap without competition. Finally, there is rarely more than 1 lobster in the kitchen. Lobsters eating the bait tend to prevent other lobsters from entering.
 
Hypothesis 1. Trap saturation is due to a balance of lobsters entering and leaving a trap. Competition at the entrance to the kitchen and between lobsters in the kitchen and outside the kitchen limits the rate of entry so that it closely matches exits. As a result, despite the large number of lobsters that encounter and enter a trap, only a few are ultimately captured.
 
Hypothesis 2. Traps first fill with small lobsters and then larger lobsters and once larger lobsters are in the trap, they limit entry.
 
Methods
 
Probably the major accomplishment not discussed so far was developing a reliable and easy to use LTV system. Currently, it is lighter, cheaper and lasts longer than the prototype. We can now collect data for two full days and we can now collect good data at night as well as during the day. We have added red lights, that the lobsters cannot see, and we supply them with power from a separate battery pack. These new modifications should enable us to collect much better and more reliable data in the future.

Publications

Available from the National Sea Grant Library (use NHU number to search) or NH Sea Grant

Journal Article

  • Watson III, W., A. Vetrovs and W. Howell (1999). Lobster movements in an estuary. Marine Biology 134:65-75.
  • Howell, W., W. Watson III and S. Jury (1999). Skewed sex ratio in an estuarine lobster ("Homarus americanus") population. Journal of Shellfish Research 18(1):193-201.

Thesis/Dissertation

  • Jury, S. (1999). Behavioral and physiological responses of the lobster, "Homarus americanus," to temperature: a new synthesis. Doctoral Dissertation, University of New Hampshire.