Characterizing Populations of Large Omnivorous Shrimp (Macrobrachium carcinus) in Puerto Rican Streams

Sarah Redd, Department of Wildland Resources, Utah State UniversityResearch Experience for Undergraduates, Program in Water Research at Colorado State University

Abstract

 Puerto Rican watersheds support 10 species of amphidromous shrimp. These shrimp use brackish water in estuaries during their larval stage then migrate into rivers and small streams. Macrobrachium carcinus is the largest shrimp species found in Puerto Rico, and is said to be delicious by local fishermen. Using three bait types and two trap types, I determined that large, local artesan traps are most effective for capturing M. carcinus when baited with cat food or chicken. Eighty-five M. carcinus were captured in three areas of the Río Espíritu Santo watershed, many, multiple times. Brightly colored alpha-numeric Visual Implant tags injected on the ventral side of the tail were prone to loss and proved to be ineffective in marking this species. Using mark-recapture data, we estimated population size to be 125 in the trapped area of the Río Espíritu Santo, 196 in the Girl Scout Stream, and 57 in Quebrada Toronja. Little movement of shrimp was observed over the course of the study, and most recaptures occurred in the same pool as the original capture. The average size of captured individuals was significantly larger in the main river than in the two other trap sites located in smaller streams, prompting questions for further study.

Study Objectives

1. Determine effective trapping methods for Macrobrachium carcinus

2. Determine the effectiveness and longevity of Visual Implant tags used for marking Macrobrachium carcinus

3. Determine preliminary population parameters for Macrobrachium carcinus, including population size, movement, and size class distribution

Introduction

  Tropical environments are home to a host of diadromous species that spend different life stages in fresh and salt water (Covich and McDowell 1996). Located in northeastern Puerto Rico, the Río Espíritu Santo is important for human recreation, water supply, and aquatic species.

http://www.lib.utexas.edu/maps/puerto_rico.html

 

Methods

Trap Efficiency

• Two trap types: a commercially purchased wire mesh gee minnow trap (4.5 cm diameter opening), and a larger, cylindrical trap built by a local fisherman (12 cm diameter square mesh opening)

• Three bait types: cat food, burnt coconut (recommended by local fishermen), and chicken

• All trap types were set in 3 pools for 11 nights, resulting in 33 replicates of each treatment

Population Data

• Traps were set in the Río Espíritu Santo and two of its tributaries, Quebrada Toronja and the Girl Scout Stream

• Captured Macrobrachium carcinus were measured to determine carapace length, weighed and marked (see tag retention section)

• Location of each capture was recorded, and data was analyzed using Microsoft Excel

Two trap types were used: modified gee minnow traps (left), and larger mesh traps made by a local fisherman (right)

Measuring the length of the carapace

Tag retention

• We injected two brightly-colored alpha-numeric Visual Implant (VI) tags in the skin on the telson just above the uropods

• We also punctured the uropods and telson with a safety pin in individual patterns, which are only expected to last a few months due to molting by shrimp

This Macrobrachium carcinus has been injected with tag V27

Average carapace length (α = 0.05, df within groups = 115, F = 23.69, P-value = 2.42 x 10-9) and mass (α = 0.05, df within groups = 93, F = 16.025, P-value = 1.05 x 10-6) are significantly larger in the Río Espíritu Santo than in the two tributaries.

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Results

Trap Efficiency

Fifty-six Macrobrachium carcinus, 88 crabs, and 12 eels were caught in the trap efficiency study. Sixty-seven percent of M. carcinus captures occurred in large traps. Traps baited with cat food or chicken parts captured a significantly higher number of shrimp than those baited with burnt coconut. Crabs were captured more frequently than M. carcinus overall, with 55.3% of crabs caught in traps baited with chicken parts.

Captured Macrobrachium carcinus by trap and bait type Captured crabs by trap and bait type

Tag Retention

In the three streams, a total of 127 individual Macrobrachium carcinus were marked with a punctured tail pattern, a VI tag, or both. Twenty-two of these were recaptured, some multiple times, resulting in 34 recapture events. Eleven of the recaptured individuals still had both a VI tag and puncture marks. In 100% of the recaptured shrimp, at least one tag was lost or unreadable. In 63.6% of recaptured shrimp, neither tag was identifiable. Tags were lost an average of 12 days after the original capture.

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Stream Avg. Carapace Length

Avg. Post-orbital

Carapace Length

Avg. Mass

Río Espíritu Santo

68.83 mm 49.97 mm 79.68 g

Girl Scout Stream

54.21 mm 39.13 mm 42.42 g

Quebrada Toronja

54.42 mm 41.29 mm 35.41 g

Average measurements for captured Macrobrachium carcinus by stream

Tag retention in recaptured Macrobrachium carcinus originally marked with 2 tags each

Stream Number marked

Number recaptured

Population estimate

Río Espíritu Santo

37 11 125

Girl Scout Stream

28 4 196

Quebrada Toronja

20 7 57

Macrobrachium carcinus

Population Parameters

To estimate population size (x) for the sample areas, I used the equation:m rx m=

where m is the number of marked individuals, r is the number of recaptured, marked individuals, and x is the estimated total population size.

Of the 22 individuals that were recaptured, 13 were found in the same pool each time. Of the 9 individuals that moved, none moved more than two trapped pools away from their original location; observed movements were both up and downstream. Individuals that moved were larger on average than those that remained in the same pool, but this difference was not statistically significant.

Conclusions

•Researchers should use large, fisherman-made traps to sample Macrobrachium carcinus as opposed to smaller minnow traps. Cat food is probably the best bait when considering both ease of use and effectiveness because it comes in small, pre-measured pouches and requires minimal preparation.

•A new method of marking these shrimp needs to be developed because VI tags are difficult to read and are often lost. Future researchers might try color-coded wire or pit tags. Marking the uropods with a safety pin may be effective for short-term studies, especially when combined with other observations to ensure correct identification.

References

 Covich, A.P. & McDowell W.H. (1996) The stream community. The Foodweb of a Tropical Rain Forest (eds D.P. Reagan and R.B. Waide), pp.433-459. University of Chicago Press, Chicago, IL.  Crowl, T.A. & Covich, A.P. (1994) Responses of a Freshwater Shrimp to Chemical and Tactile Stimuli from a Large Decapod Predator. Journal of the North American Benthological Society, 13(2): 291-298.  March, J.G., Benstead, J.P., Pringle, C.M., & Scatena, F.N. (1998) Migratory Drift of Larval Freshwater Shrimps in Two Tropical Streams, Puerto Rico. Freshwater Biology, 40: 261-273.  March , J.G. & Pringle, C.M. (2003) Food Web Structure and Basal Resource Utilization along a Tropical Island Stream Continuum, Puerto Rico. Biotropica, 35(1): 84-93.  March, J.G., Pringle, C.M., Townsend, M.J., & Wilson, A.I. (2002) Effects of freshwater shrimp assemblages along an altitudinal gradient of a tropical island stream. Freshwater Biology, 47: 377-390.  Greathouse, E.A., March, J.G., & Pringle, C.M. (2005) Recovery of a Tropical Stream after a Harvest-Related Chlorine Poisoning Event. Freshwater Biology, 50: 603-615.  Greathouse, E.A., Pringle, C.M., McDowell, W.H., & Holmquist, J.G. (2006) Indirect Upstream Effects of Dams: Consequences of Migratory Consumer Extirpation in Puerto Rico. Ecological Applications, 16(1): 339-352.  Kartchner, Summer C. (2002) Recreational Use of Montane Streams of the Puerto Rican Rainforest. Unpublished report.

One of the trapped pools on the Río Espíritu Santo

One of the many crabs that inhabits this watershed

Macrobrachium carcinus is both the largest and the most sought after species by fishermen. Fisherman use traps, hooks and lines, nets, spears, spear guns, and poison (such as bleach) to capture these large decapods (Greathouse et al. 2005, Kartchner 2002) for personal consumption.

Urban development is rapidly increasing in Puerto Rico. As the human population grows, it is important to determine potential effects of fishing and other recreational activities, some of which have already impacted shrimp populations in Puerto Rico (Greathouse et al. 2006). Efforts to quantify these populations and their interactions with the system as a whole will be useful in assessing future impacts as well as furthering current scientific knowledge.

•Though it appears that large Macrobrachium carcinus are resident to specific pools, the mark-recapture study should be continued for a longer time period and expanded over a larger area to increase precision. Similarly, more M. carcinus should be marked in order to increase confidence of population estimates. Future questions for study could focus on the reasons for migration in this species or to determine why some individuals remain in the main river while others migrate higher into the small tributaries.

Crabs and M. carcinus were most often captured singly in a trap, suggesting that trap choice may be influenced by the presence of other large individuals. One hundred ninety-eight total traps were set in the three pools used for the trap efficiency study; M. carcinus were captured in 46 of those (23.2%).

Many thanks to Katie Hein, Todd Crowl, and Therese Frauendorf, whose advice and assistance made this project possible. The Research Experiences for Undergraduates Program in Water Research at CSU was funded by a grant from the The DoD's ASSURE (Awards to Stimulate and Support Undergraduate Research Experiences) Program in partnership with the National Science Foundation.

Three genera of decapod shrimp have been observed in Puerto Rican watersheds. Typically, adult shrimp carry their eggs while inhabiting high mountain streams. The eggs then hatch and the larvae are transported downstream into estuaries where they undergo metamorphosis into adult morphs. After metamorphosis, these larvae migrate upward into rivers and streams where they spend the majority of their lives (March et al. 1998).  

M. carcinus alone Crab alone Multiple M. carcinus

Multiple crabs Crab(s) with M. carcinus

29 traps 29 traps 6 traps 16 traps 11 traps

31.8% 31.8% 6.6% 17.6% 12.1%