Queen Conch Survey

How is it funded, who administers it?


Why was the survey originally designed?


What are/were the intended outcomes/objectives?


Describe the temporal and spatial coverage of the survey

Show the geographic coverage (including depth) of the survey (maps are helpful)

How often is the survey conducted (annually, biennially, biannually etc.)?

Is it seasonal?


Describe the underlying experimental design

Fixed site or stratified-random (If stratified-random, describe the strata)?

Describe allocation of sites per strata (proportional/weighted? ) and number of sites


Describe the methodology and gear


Describe the outputs of the survey

Describe the data outputs & include a discussion of the variance structure of the data


Self-Evaluation of pros and cons of survey

What suite of species does this survey target (what are gaps)?

What are the gear/method biases (catchability, size selectivity)?  

What are the temporal/spatial limitations?

Is this survey expected to continue into the future?

It is relatively expensive/inexpensive, logistically difficult/easy?


Self-Evaluation of utility of survey for generating information for stock assessment

Spatial/Temporal coverage

Data generated: length frequency, spawning stock biomass, mortality etc.


Provide any other information that may be relevant to this discussion


Provide most relevant documentation (w/PDFs or hyperlinks if possible)


Approach (Procedures):


During this year the Queen's conch, Strombus gigas, resources surrounding Puerto Rico will be surveyed. The following methodology will be used:

  1. Queen conch fishermen along the west, east and south coast of Puerto Rico will be interviewed about their fishing grounds.  Information collected in the interview will include fishing history on the area to separate new fishing grounds from those with a greater fishing pressure.  This information will be compared with the data collected for the queen conch stratification survey done on 1995 (Rosario, 1995).  The same questionnaire will be used.
  2. Sample stations will be selected in a stratified randomized manner. Stratification will be based on expected abundances as determined by historical fishing patterns. A minimum of 100 stations will be targeted.
  3. Prior to the start of the survey divers will be trained on the use of the scooters; to identify and measure live conch; and maintain speed, direction and transect width.  They will also be briefed on safety rules.
  4. Transects are expected to be conducted during the closure season for the queen conch, from 1 July to 30 September of each year.  This will prevent to a certain degree doing transects in places recently fished out.  Also we want to avoid spreading the surveys during a whole year given that queen conch grow fast during their first two years (Appeldoorn).
  5. Abundance and density of queen conch will be estimated from visual census surveys conducted along transect by Scuba divers using underwater scooters. Maximum survey time will be 45 minutes and will not exceed the no-decompression limits. Differential Global Position Systems will be used to locate the beginning and end of each transect. One of the divers will carry the Scuba safety buoy.  The other diver will carry the compass to follow a fixed direction for a set period of time. Depth, habitat type, start and end time, time at each habitat change will be recorded. While conducting transect, the scooter will be kept approximately one meter above the substrate so that path width remained constant at 4 meters within the transect. All conchs will be counted.
  6. The length of the individual conch will be measure to the nearest cm; and adult age will be estimated to one of the four relative age classes (newly mature, adult, old adult, and very old adult). Record will be kept of time when each individual is found and time when the survey is resumed. Habitat types will be the following: sand, coral, hard ground, gorgonians, seagrass and algal plains.
  7. Length of transect will be obtained by calculating the distance between the beginning and end points of each transect. Using this information, speed will be calculated in meters/minute. Total area will be calculated by multiplying the distance of each transect by the transect width (4 meters). Densities will be calculated by summing the number of an adult conch observed per transect divided by the total area of each transect. Densities for each habitat will be derived by dividing the number of conch per habitat type by the total area of that habitat type per transect. Densities based on depth will be calculated by determining the number of conch in each depth range divided by the time and speed over that depth range. Overall abundance will be estimated. Length (juvenile, adult) and age (adult) frequencies will be determined. Differences in density or abundance by habitat, depth, or location will be statistically tested.
  8. Length frequency distribution for adults and juveniles respectively are created. Juvenile length frequency distribution was analysed using the Bhattacharhya’s method of the FISATII program. 


Approach 1995

Stratification was based on expected abundance as determined by historical fishing patterns.  In each area sampled (West and East coasts) two strata were defined.  One consisted of the area identified to be areas of current or former fishing activity or areas known to have juveniles or other adults but not fished.  The second stratum was all areas not identified as having or had conch.  Within each stratum, stations were selected randomly from a grid set at 0.1-min latitude b 0.1-min longitude.  One hundred stations were targeted.  Areal emphasis for the survey was the southwest insular shelf of Puerto Rico, but some sampling was reserved for the east coast shelf.

Estimates of abundance and density of queen conch were made from via\Sal surveys along strip-transects.  Surveys were conducted by divers using underwater scooters.  Transect width was four meters.  Transect length was variable based on depth, but maximum survey time was set at 45 minutes, and no dives exceeded the no-decompression limits for diving safely.  At each station, parallel transects were made (one/diver).

Global Positioning System was used to locate the beginning and the end of each transect. A buoy was dropped at the starting point of each transect, from which divers followed a fixed compass heading for a set period of time, the latter determined by depth.  Prior to conducting a transect, a four-meter long marker was placed on the bottom to calibrate transect width.

For each transect, depth and strata time was recorded.  While conducting a transect, the scooter was kept approximately one meter above the substrate so path width remained constant at 4 meters.  All conch were counted.  The length of all individuals were estimated to the nearest 1 cm, and if an adult its age was estimated to one of four relative age classes based on the degree of shell erosion: newly mature, adult, old adult, very old adult.  Definitions of these are given in Table 1.  Records were kept of habitat type, depth, time over each habitat type and depth, and time of appearance of each conch observed. Habitats types were recorded using classes based on sediment characteristics or dominant biota: Sand, Gorgonian, Thalassia, Halimeda, Halophila, Mud, Coral, Hard Bottom, and Rubble.  Combinations of these were used to classify areas of mixed habitat.  For analysis habitats were grouped into six categories: sand, mud, coral reef, coral rubble, seagrass and algal plain.

Transect length was obtained by calculating the distance between the beginning and end points of each transect. Total area was calculated by multiplying the distance of each transect by the transect width. Area for each habitat was calculated by multiplying the total area by the percentage of time spent over each habitat.  Densities for each habitat were derived by dividing the number of conch per habitat type by the total area of that habitat type per transect.  Overall abundance was estimated from the data on density/station.  Length (juvenile, adult) and age (adult) frequency distributions were determined, differences in density by habitat were tested using the Kruskal-Wallis non-parametric analysis.

Prior to the start of the actual survey, divers were trained to identify live conch, maintain speed and transect width, and to estimate length and adult age-classes.  A reference collection of adult conch for each age group was maintained.

2013 Approach

Visual Surveys

In 2006, interviews with fisherman identified areas of past conch fishing grounds, present conch fishing grounds and areas known to have juveniles on the west, east and south coasts. The west coast is the primary fishing grounds for queen conch in Puerto Rico.  These maps were digitized into a GIS database using ArcMap and used as boundaries to create 46 random survey sites off the west coast, within the 90-ft contour, using the “create random points” tool (Figure 3).  The 90-ft boundary was chosen for diver safety.  All categories (past, present and juvenile) were given the same weight during the site selection, though many of the polygons overlapped. 

Methods for this survey were kept identical with previous years’ surveys to facilitate comparison of results between surveys.  All divers participating were trained in the following: identification of Strombus gigas, use of the underwater scooters including maintaining constant direction and speed as well as safety protocol, estimating lengths and identifying age classes using an established reference collection, completing practice transects and recording all applicable data.       

At each of the sites, paired visual surveys were done on SCUBA with the help of underwater scooters to maximize distance travelled.  Each diver surveyed a 4m wide transect of variable length depending on depth and available dive time, but for a maximum of 45 minutes.  One diver carried a safety buoy which helped identify the transect end point and allowed the surface support vessel to track the divers; the other diver carried a compass set to a fixed random heading so the dive pair could follow a straight line.  During the survey, habitat, depth, age class and estimated length were recorded for each conch, as well as observations of copulation or egg laying.  Classifications of habitat included sand, gorgonians, Thalassia, Syringodium, Halimedia, algae, reef, hard bottom or any combination of these.  Age classes were juvenile (J), newly mature adult (NMA), adult (A), old adult (OA) and very old adult (VOA) and are classified based on the characteristics found in Table 3.  Transect distance was calculated in ArcMap by measuring the straight line distance connecting the start and end positions. 


Data Analysis

Total area surveyed was calculated by multiplying the length of the transect by 4m width and then doubling the area (two transects per site) and finally summing over all 46 sites (92 transects).  Densities were calculated by dividing number of conch observed at each site by the area surveyed.  Comparisons of densities of both adults and juveniles between years (1997, 2001, 2006 and 2013) were made by modeling densities as a function of management regime (territorial or federal), depth, habitat and year using a log transformed negative binomial distribution.  Analyses were conducted using the generalized linear model function (GLIMMIX) of SAS. This distribution was chosen over a Poisson because it is better equipped to handle high variability.  No spatial correlation term was included in the model because the inclusion of the depth and habitat terms explained most of the variability.  Including the management regime in the model helped to elucidate the effectiveness of a more than 10 year closure of the fishing grounds in the federal area.  Trends regarding age structure and size frequency were also described.  The spawning stock for the west coast was calculated using only the older age classes (adult, old adult and very old adult) densities multiplied by estimates of suitable habitat area on the western platform based on the previously digitized strata.  This spawning stock was then compared to the mesophotic population estimate at Abrir La Sierra (Garcia Sais et al. 2012) to get an idea of the potential contribution of the mesophotic population relative to the shallow water stock.

Figure 1: Position of Abrir La Sierra relative to the western insular shelf of Puerto Rico. (From Garcia-Sais et al. 2012).


Figure 2. Nautical chart with plots of the starting point of the queen conch survey transects. 2006


Figure 3: Location of random sample sites for the 2013 conch visual surveys relative to the mainland of Puerto Rico.