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An Ongoing Study of the Relationship between Net Type and the Size and Physical Condition of Captured Cnidarian Specimens
Introduction
Question
Alexander Hurley1, 2, Kelly Robinson, Ph.D.1, Monty Graham, Ph.D.1 1Department of Marine Science, University of Southern Mississippi
2National Oceanic and Atmospheric Administration (NOAA)
AcknowledgementsI want to thank Dr. Matthew Klooster for his networking assistance, Dr. Graham and Dr. Robinson for hosting me at the University of Southern Mississippi and for mentoring me throughout the summer, the Oregon II crew for their patience and diligence at sea, the NOAA scientists aboard the Oregon II for their encouragement, and James Graham Brown Foundation for their financial support and continuous vote of confidence in me.
ReferencesGraham, W.M. (2001). “Evidence for numerical and distributional changes of jellyfish populations in the northern Gulf of Mexico.” In J. Purcell, W. Graham and H. Dumont (eds.) Jellyfish Populations: Ecological and Economic Effects. Kluwer Academic. Submitted. National Oceanic and Atmospheric Administration, Southeast Fisheries Science Center, Mississippi Laboratory. “Resource Assessment Surveys Cooperator Information.” Pascagoula, MS.
Results
Conclusions and Future Work
Discussion
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Figure 1. Cnidarian size distribution in mm for 19 specimens captured in the Bongo net. Average size is 36mm. Without the two largest data points, the average comes to 25mm.
Figure 2. Cnidarian size distribution in mm for 24 specimens captured in the Neuston net. Average size is 73mm.
Figure 3. Cnidarian Size distribution in mm for 2 specimens captured in the trawl net. Average size is 65mm.
Figure 4. Cubozoan size distribution in mm for 15 specimens captured in the bongo net. Average size is 33mm.
Too few cnidarians were captured in the standard trawls to use for analytical purposes, so those data points will be omitted from this discussion (see Figure 1). A comparison of the cnidarians captured in bongo and neuston nets appears to be worthwhile for this qualitative discussion because the number of data points are relatively similar, being 19 and 24, respectively (see Figures 2 and 3). The average size of the cnidarian specimen caught in the neuston net was 37mm larger than the average cnidarian specimen caught in the bongo net. If the two potential outlier points (145mm and 110mm) for the cnidarian-bongo data are removed from the averaging process, then the disparity between the average cnidarian size caught in the neuston versus the bongo increases to 48mm. There are several explanations for why the neuston net may tend to capture larger cnidarian specimens than the bongo net. The first being the depth at which these two nets operate. The neuston nets stay on the surface for the duration of their deployment, whereas the bongo nets dive to within two meters of the bottom with a max depth of 200 meters. For one reason or another, larger cnidarians may tend to be located closer to the surface whereas smaller cnidarians may tend to be located deeper in the water column. This could take place for a variety of reasons, including water currents, locomotion, salinity, time of day, etc. Another explanation for this disparity could be related to characteristics of the nets themselves. It could be that the neuston nets do a better job of maintaining the physical condition of larger specimens, and so there was an increased probability that if the neuston net caught a large specimen that that individual would be in good enough physical condition to successfully record its size. Vice versa, the bongo net could have simply done a better job of collecting and maintaining smaller specimens up until their measurement.
From the analysis of the initial data, cubozoans were only captured in the bongo net (see Figure 4). This can be explained for several reasons. First, because only 15 data points were recorded, it could be that too few data points were collected to show a normal representation of the capturing rate of the different nets (this argument can be applied to the cnidarian data too). Second, the bongo net could have more effectively captured cubozoans than the other two nets. The argument was made above that the bongo net could have the unique ability to capture smaller gelatinous zooplankton, and because the average size of the cubozoans in the bongo was similar to the average size of the cnidarians in the bongo, this could support that argument. It could also be that cubozoans are only found at lower depths than at which the neuston net operates. The gaps in the standard trawl net are almost certainly the reason that no cubozoans were found in that net type.
From a qualitative analysis perspective, the neuston net tends to capture larger gelatinous zooplankton specimens than the bongo net. This could be do to the variables of depth, net characteristics, or to others. This argument is tenuously supported by the fact that the recorded average size for the cubozoans was similar to the recorded average size for cnidarians. There is simply not enough data on the standard trawl net to make any speculations about a relationship between this net and specimen size.
Future work includes working with larger data sets that are available from NOAA research labs, which will allow for a robust quantitative analysis of the relationship between specimen size and net type. Additionally, a physical condition index could be created in order to give each of the photographed specimens a physical “score” after being removed from the net. From there, an analysis of the relationship between the physical condition score and the net type could be conducted.
Materials and Methods
Image 1. On the deck of the Oregon II. Image 2. NOAA Ship Oregon II anchored in Galveston, TX.
Image 3. Map of the western Gulf of Mexico. Pins identify the stations where gelatinous zooplankton specimens were gathered. The numbers indicate the station number of each of these fishing stations. Note Galveston in the upper left and New Orleans in the upper right as reference points.
While in-depth scientific studies of jellyfish and fellow members of the Cnidarian phylum are lacking relative to other areas of marine research, these creatures are still widely viewed as important components of marine ecosystems for a variety of reasons. One reason for their ecological importance is the role they play in marine food webs - cnidarians often compete directly with fish and other organisms for zooplankton as a source of food. This, coupled with the propensity of Cnidarian populations to periodically form blooms (a rapid increase in population size) makes this a group worthy of regular ecological surveys. Data collection on these gelatinous creatures is not always an easy task though, as there is not a reliable way to capture them on a broad-scale that also ensures that they will be physically intact upon completion of their capture. Nets designed for fish and plankton are the current standard used to complete the task, but the specimens that ultimately make it into the boat are often physically compromised in one or more ways. This study sought to collect data on cnidarian specimens caught in the Gulf of Mexico in different fishing and plankton nets in order to demonstrate whether or not there is a relationship between the type of nets used and the size and/or physical condition of the cnidarian specimens.
Does the type of fishing/plankton net relate to the size and/or physical condition of the cnidarian specimen captured by that net?
Collecting Specimens:The sea vessel used to navigate the Gulf and to capture the specimens via the different fishing and plankton nets was the National Oceanic and Atmospheric Administration’s Oregon II (see Image 2). This study was conducted during the second leg of the Oregon II’s Summer Groundfish Survey, which spanned the northern Gulf from Galveston, TX to Tampa Bay, FL. Stations (locations to cast out fishing or plankton nets) are randomly chosen throughout the Gulf prior to the trip, with depth ranging from 5-60 fathoms (see Image 3). The standard trawling net was conducted from one of two rear outriggers with a 40 foot shrimp trawl. The trawl is towed for no less than 10 but not more than 50 minutes (see Image 4). The plankton nets included bongo and neuston samplers. The bongo samplers are cone shaped nets of 0.333 micron mesh webbing attached to two circular 61 centimeter diameter frames that are designed to capture plankton throughout the water column. Bongo casts are done off the J-frame on the port side of the forward deck of the ship. They are lowered through the water column at an angle of 45 degrees to a depth within 2 meters of the bottom (see Image 5). Neuston tows collect plankton on the surface in a net with an opening of 1x2 meters. This net is deployed off of the starboard side of the forward deck with the forward crane of the ship (see Image 6).
Image 4. Standard trawling net off the starboard side of the boat.
Image 5. Bongo nets.
Image 6. Neuston net.
Image 7. Cnidarian specimen. Image 8. Cubozoan specimen.
Processing Data:Data processing took place at the Department of Marine Science at the University of Southern Mississippi. The first step of this process was to identify the specimens based off of the digital photographs collected. Unfortunately, many of the specimens were in such poor physical condition or were otherwise difficult to discern at the species or genus level. Therefore, the label of cnidaria (taxonomic level of phylum) was assumed for all those specimens with the traditional jellyfish bell structure in order to catch all of the species present. For those species that did not have the traditional bell structure, the label of cubozoan (taxonomic level of class) was given. Histograms containing size data points for a given net type were created for these two categories (cnidaria and cubozoa) in order to qualitatively analyze the relationship between size and net type, if any. A quantitative analysis of this relationship between size and net type has yet to be performed, as more data need to be added to the analysis before proceeding. The relationship between the physical condition of the specimens and the net type has yet to be analyzed as well.
Collecting Data:Once aboard the ship, the cnidarian specimens underwent a multi-step data collection process. For the purposes of this study though, only size and physical condition data were utilized. The tool used to measure specimen size was an electronic measuring board that rounded to the nearest millimeter. The technique for measuring the size of the cnidarian depended on the organism in question. For those cnidarians with “traditional” jellyfish bells, the bell diameter was used to measure size. For those cnidarians which fell into the cubozoan class (e.g. Tamoya haplonema), then oral-aboral length was utilized for the size measurement. In order to collect data on the physical condition of the specimens, a digital camera was utilized to photograph the specimen on a work bench. Again, the technique for collecting this data depended on the organism in question. Those specimens with traditional jellyfish bells were splayed out with both their tentacles (or what was left of the tentacles) facing up and facing down in order to gather multiple views of the condition of the organism (see Image 7). For those specimens in the cubozoan class, the organism was laid so that the side of its bell was flat on the bench, with tentacles facing one way and the top of the bell facing in the other direction (see Image 8).
