From: http://www.nefsc.noaa.gov/esb/why_nefsc_surveys.htm#neo7
Why:
- Monitor recruitment: they use nets that catch very small fish, allowing them to predict the number of young fish, which is used to give predictions of upcoming seasons.
- Monitor abundance and survival of harvestable sizes: information from fish in year x can be compared with information in year x-1 to see how the population has changed; the results will indicate mortality rates. (? If mortality rates are drastically different than expected, does this indicate a high level of illegal fishing?)
The estimate population by using the equation: average of: (number caught in sample) / (area of sample) for each sample, then multiply by total area of stock
- Monitor geographic distribution of species: shows how stock distribution moves.
- Monitor ecosystem change: each bottom trawl survey yields data for up to 200 species. Can see how fishing a few species impacts the other species
- Monitor biological rates of stocks: data shows growth rate, sexual maturity rates, feeding rates. Changes in these values predict changes in stock size.
- Collect environmental data/Support other research: surveys are conducted 24 hrs/day while at sea. Crews can monitor pollution, temperature, etc
Types of Surveys:
- Spring/fall bottom trawl: survey from 5 to 200 fathoms from Cape Hatteras (N Carolina) to "well beyond" Canadian border. 300 half hour trawls selected through a stratified random sample. Only two boats have been used.
- 1/2 inch net used to capture recruits.
- All species in each tow are counted and weighed, and all or a subsample are measured for length
- Some hard parts (scales, ear stones, fin rays) are taken from a smaller sample (these parts determine the age)
- Age of whole stock is estimated from this sub-sample
- Checked for gender and sexual maturity
- Stomach contents identified, disease inducing agents recorded
- All the data is available to scientists, which means that there should be a way for us to get it.
- Sea Scallop survey: continental shelf from Cape Hatteras through Georges Bank and Gulf of Maine
- Stratified Random Sample
- 8ft wide commercial scallop dredge with 2 inch ring abg and 1.5 inch mesh liner.
- 15 minute tow
- 15 to 60 fathoms
- Large amount of bycatch; it is used to collect info on the bycatch species (similar to spring/fall trawl I suppose...the article doesn't say much though)
- Hydraulic Clam Dredge: survey for surfclams and ocean quahog. Surveyed ever three years.
- 5 foot wide dredged is used; it has a submersible pump that uses high pressure water to pry the animals off of their habitat
- The can survey deeper than commercial fishing equipment can go
- The 2 inch nets keep small clams along with a lot of shell debris, live clams, and small invertebrates (all of which are documented)
- Survey from Cape Hatteras to Georges Bank, with some surveying in Mass. Bay and the Maine coast
- Gulf of Maine summer trawl: special bottom trawl focused on near-shore areas.
- Has traditionally been impractical to include Maine, New Hampshire, and N Mass waters in spring/fall surveys because the sea floor is too rocky and there are too many lobster pots/gill nets.
- Modified Stratified Random sample--they compiled a list of all the locations in the survey area that could be trawled (using knowledge from fishermen, etc). Sites are randomly chosen from this list.
- Has occurred since 1991
- Important because there are large concentrations of young fish that were not accounted for in the fall and spring surveys
- Winter trawl along continental shelf: trawl along middle atlantic, S New England, and Georges Bank to improve data quality for flatfish (flounders, soles, halibut, http://en.wikipedia.org/wiki/Flatfish)
- A chain sweep (a flat net) is used to catch the fish (previous methods used a foot rope and rollers, but too many fish escaped under the rollers and threw off the results)
- Began 1992
- Marine Mammal Sighting surveys: a ship is directed along a predetermined (? probably random?) trajectory, and the number and type of marine mammals seen is recorded
- complicated interpretation methods (less likely to see an animal that is present if it is far away, etc)
- Began 1991
- Survey of fish eggs and larvae: surveys to estimate the number of eggs and new hatchlings
- Several times a year
- Indicates spawning season/location
- Also indicates number of new fish, which can be used to estimate number of spawning females
- Very fine mesh nets are towed in a frame (bongo nets), they stretch from the surface to the sea floor
- The contents are all preserved for intense analysis in labs
- free floating plants and animals are also captured, which helps estimate plankton distributions
- There are also special surveys that are conducted to test how effective different methods are, and to test new equipment (e.g. they had to change trawl net doors on the boats, and they needed to know how the new doors would change the catch so that new information could be related to old data)
Why do they sample where they do?
- Random samples--study statistics
- There are some countries that used fixed sampling sites--I'm not sure if they mean that they sample the same sites each time (using the above methods), of if they have some sort of fixed device (as we discussed on Friday)
There are some articles that they list that provide more information--some of these might be useful:
Azarovitz, TR. 1981. A brief historical review of the Woods Hole Laboratory trawl survey time series. In: Doubleday, WG and Rivard, D, eds. Bottom trawl surveys. Canadian Special Publication of Fisheries and Aquatic Sciences 58, p. 62-67.
Clark, S. 1981. Use of trawl survey data in assessments. In: Doubleday, WG and Rivard, D, eds. Bottom trawl surveys. Canadian Special Publication of Fisheries and Aquatic Sciences 58, p. 82-92.
International Council for the Exploration of the Sea. 1992. Report of the workshop on the analysis of trawl survey data. ICES C.M. 1992
:6.