SEAMAP Surveys

PLANKTON SURVEYS

Plankton samples are taken at stations arranged in a systematic grid across the Gulf of Mexico. Such a grid is chosen because of the large survey area. Stations are set at minimum intervals of 30 miles (2 degree) and during the Fall Plankton Survey, Mississippi samples stations set at intervals of 6 nautical miles.

Plankton sampling gear consists of standard 61-cm bongos and a 2x1-m neuston net for the large vessels. The bongos are fitted with 0.333-mm mesh nets with either hard (PVC) or soft (0.333-mm mesh net) cod ends. The neuston net consists of a 2x1-m pipe frame fitted with a 0.948-mm mesh net on which the cod end is tied off.

At each designated plankton station, either an oblique bongo and surface neuston tow or a surface neuston tow is made. At bongo stations a standard oblique tow is made to 200 m, or to 2 m off the bottom at depths less than 200 m. Neuston tows are made at the surface with the net half-submerged for 10 minutes at a vessel speed of 1.5 knots.

All ichthyoplankton components (eggs and larvae) are removed from each sample and the fish larvae identified to the lowest feasible taxon (families in most cases).

Spring Plankton Survey

The objectives of the Spring Plankton Survey are to collect ichthyoplankton samples for estimates of the abundance and distribution of Atlantic bluefin tuna larvae and collect environmental data at all ichthyoplankton stations. Plankton samples are taken with standard SEAMAP bongo and neuston samplers. In addition, hydrographic data (surface chlorophylls, salinity, temperature and dissolved oxygen from surface, midwater and near bottom, and Forel-ule color) are collected at all stations.

Fall Plankton Survey

The objective of the Fall Plankton Survey is to collect ichthyoplankton samples with bongo and neuston gear for the purpose of estimating abundance and defining the distribution of eggs, larvae, and small juveniles of Gulf of Mexico fishes, particularly king and Spanish mackerel, lutjanids and sciaenids. Plankton samples are taken with standard SEAMAP bongo and neuston samplers. In addition, hydrographic data (surface chlorophylls, salinity, temperature and dissolved oxygen from surface, midwater and near bottom, and Forel-Ule color) are collected at all stations.

ENVIRONMENTAL DATA

Standardized methodology is used although the actual parameters measured varied among vessels participating in each survey. These parameters are measured based on equipment availability. The following parameters are recorded:

Vessel: Vessel code for each vessel.
Station: Station identifiers vary by state and vessel.
Cruise: Cruise numbers vary by state and vessels.
Date: Month/Day/Year.
Time: Local time and time zone, recorded at the start of sampling.
Latitude/longitude: Recorded to seconds.
Barometric pressure: Recorded in millibars.
Wave height: Estimated visually in meters.
Wind speed and direction: Recorded in knots with direction recorded in compass degrees from which the wind is blowing.
Air temperature: Recorded in Centigrade.
Cloud cover: Estimated visually in percent cloud cover.
Secchi depth: Secchi depth in meters, estimated at each daylight station. Standard oceanographic 30-cm white discs are lowered until no longer visible, then raised until visible. If different depths are recorded, an average is used.
Water Color: Forel-Ule data is recorded.

The following parameters are measured at the surface, mid-depth and bottom; for bottom depths greater than 200 m, samples are taken at surface, 100 m and 200 m:

Water temperature: Temperatures are measured by a hand-held thermometer or by in situ electronic sensors onboard ship.
Salinity: Salinity samples are collected by Niskin bottles and stored for laboratory analysis with a salinometer. Conductivity probes or refractometers are used on some vessels. Salinity samples are also measured with in situ electronic sensors.
Chlorophyll: Chlorophyll samples are collected and frozen for later laboratory analysis. Laboratory analyses for chlorophyll a and phaeophytin a (a chlorophyll degradation product) are conducted by fluorometry and spectrophotometry. In addition, chlorophyll data are also collected using a CTD. This method only obtains measures of chlorophyll a and is a measure of fluorescence.
Dissolved oxygen: Dissolved oxygen values are measured by electronic probes or by the Winkler titration method.
Turbidity: Turbidity values are measured by electronic probes when equipment is available.

TRAWL SURVEYS

Summer Shrimp/Groundfish Survey

Objectives of the Summer Shrimp/Groundfish Survey are to monitor size and distribution of penaeid shrimp during or prior to migration of brown shrimp from bays to the open Gulf; aid in evaluating the "Texas Closure" management measure of the Gulf of Mexico Fishery Management Council's Shrimp Fishery Management Plan; and provide information on shrimp and groundfish stocks across the northern Gulf of Mexico from inshore waters to 50 fm.

Since 1987, the strategy for the trawl surveys has been that day/night sampling sites are chosen randomly in areas stratified by depth and statistical area (statistical zones 11 through 22) and sample from the eastern Gulf to the Texas/Mexico border, in order to sample during or prior to migration of brown shrimp from bays to the open Gulf area. Trawl stations sampled by NMFS, Alabama, Mississippi and Louisiana are made with a standard SEAMAP 40-ft net, and Texas samples with a 20-ft net. Depth strata consist of 1-fm intervals from 5 to 20 fm, a 2-fm interval from 20 to 22 fm, a 3-fm interval from 22 to 25 fm, 5 fm intervals from 25 to 50 fm and a 10-fm interval from 50 to 60 fm. Additionally, the USM/CMS/GCRL vessel TOMMY MUNRO samples 1-fm intervals from 2 to 5 fm off Louisiana in July. Trawls are towed perpendicularly to the depth contours and cover the entire depth stratum on each station. Single tows are for a maximum of 55 minutes; for certain stations, a series of consecutive trawl tows is necessary to cover a given depth stratum, with a minimum individual tow across each stratum of 10 minutes and a maximum tow of 55 minutes. The Texas vessels tow 10 minutes parallel to the depth stratum.

All Penaeus spp. shrimp are separated from the trawl catch at each station. Total count and weight by species are recorded for each station. A sample of up to 200 shrimp of each species from every trawl is sexed and measured to obtain length-frequency information. Estimated total numbers are derived from the total weights of those processed. Other species of fishes and invertebrates are identified, enumerated and weighed. Weights and individual measurements on selected species other than commercial shrimp are also recorded.

Fall Shrimp/Groundfish Survey

The objectives of the Fall Shrimp/Groundfish Survey are to sample the northern Gulf of Mexico to determine abundance and distribution of demersal organisms from inshore waters to 60 fm; obtain length-frequency measurements for major finfish and shrimp species to determine population size structures; collect environmental data to investigate potential relationships between abundance and distribution of organisms and environmental parameters; and collect ichthyoplankton samples to determine relative abundance and distribution of eggs and larvae of commercially and recreationally important fish species.

The design of the fall survey is similar to the Summer Shrimp/Groundfish Survey. During the fall survey trawl stations are made with the standard 40-ft and 20-ft SEAMAP nets and cover NMFS shrimp statistical zones 11 through 21. All organisms that are caught in the trawls are treated in the same manner as the Summer Shrimp/Groundfish Survey with the exception to shrimp catches where only 20 shrimp of each species from every trawl are measured, although Louisiana measures a minimum of 50 shrimp.

REEF FISH SURVEY

The primary purpose of this survey is to assess relative abundance and compute population estimates of reef fishes found on natural reef fish habitat in the Gulf of Mexico. Two types of gear are used to deploy video cameras: 1) a single-funnel fish trap (2.13 m long by 0.76 m square) with the camera mounted at a height of 25 cm above the bottom of the trap; or 2) a 4 camera array with 4 cameras mounted orthogonal to each other at a height of 25 cm above the bottom. Both gears are baited with squid before deployment. The resultant video recordings (typically of one hour duration) are processed back at the laboratory where fishes are identified and counted independently by two tape readers. Final counts are entered into the SEAMAP reef fish database along with additional observations on habitat and fish activity.

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Last modified: February 07, 2007

SEAMAP Gulf of Mexico Resource Surveys