Blanco J.F.

Effects of coupled natural and anthropogenic factors on the community structure of diadromous fish and shrimp species in tropical island streams

CATHERINE L. HEIN*, ANDREW S. PIKE†1 , JUAN F. BLANCO‡, ALAN P. COVICH§ , FREDERICK N. SCATENA†, CHARLES P. HAWKINS* AND TODD A. CROWL. Effects of coupled natural and anthropogenic factors on the community structure of diadromous fish and shrimp species in tropical island streams. Freshwater Biology. Vol 56, Is 5 pp 1002-1015.

1. Overlapping river and road networks provide a framework for studying the complex interactions between natural and human systems, with river-road intersections as focal areas of study. Roads can alter the morphology of stream channels, pose barriers to freshwater fauna, provide easy access to streams for humans and non-native species and accelerate the expansion of urban development. 2. We determined what variables control the structure of diadromous fish and shrimp communities and assessed whether particular road crossings altered community structure in north-eastern Puerto Rico. We identified 24 sites that represented a range of river and road sizes across two catchments that drain El Yunque National Forest in Puerto Rico. 3. The location of natural barriers and the size of stream pools were the most important variables for predicting six of fifteen fish and shrimp distributions. Predatory fishes were predicted to be limited to areas in the river network below large, steep waterfalls, whereas adult shrimp Atya lanipes (Atyidae) were predicted to be present above these waterfalls. The fish Awaous banana was predicted to be present in pools >11.6 m wide, whereas the shrimp Xiphocaris elongata was predicted to be present in pools <10.4 m wide. The distributions of nine species were predicted poorly, but three of these species were common and three were rare. 4. Although urban and agricultural land covers were among the top three predictors of five species distributions, they were probably good predictors because they were correlated with the natural gradient. Further study is necessary to disentangle natural and anthropogenic gradients. 5. Road crossings, 10 of which were culverts, were not dispersal barriers for fishes or shrimps. On average, species were present both upstream and downstream from road crossings at 68% of sites where they occurred. Absences upstream or downstream from road crossings occurred at 16% of sites each and likely resulted from a failure to detect species.


BLANCO,JUAN F.;SCATENA,FREDERICK N. 2007. The spatial arrangement of neritina virginea (gastropoda: neritidae) during upstream migration in a split-channel reach.. River Res. Applic. 23: 235-245.

This paper relates differences in flow hydraulics between a main channel (MC) and a side channel (SC) of a river to patterns of upstream migration by Neritina virginea (Neritidae: Gastropoda), a dominant diadromous snail in streams of Puerto Rico (Greater Antilles). Near-bed water velocity, snail density and shell size were measured on a weekly basis between August and December 2000 along cross-sections in a main channel (MC) and an adjacent channel (SC) under a bridge crossing of the Rio Mameyes of Northeastern Puerto Rico. Near-bed velocity and water depth were used to compute Reynolds (Re) and Froude (Fr) numbers, and to classify flows within each channel. During base flow conditions (<2m3 s1), flow was chaotic and supercritical (Fr>1) in the MC, and non-chaotic and subcritical (Fr<1) in the SC. Higher mean densities (>100 indm2) of relatively small snails (mean s.d., 6.3 2.8 mm) were consistently recorded in the MC. Conversely, the SC had lower mean densities(<20 indm2) and significantly larger snails (7.6 2.4 mm). Within the MC, migratory groups preferred near-bed velocities>0.8ms1. Within the SC, they preferred the channel thalweg and depths>30 cm. The spatial arrangement that was observed between and within the channels may be related to food resources, predation pressure or biomechanics. Characteristics of preferred upstream migration pathways of N. virginea must be accounted when building road crossings incoastal streams with diadromous fauna.

Floods, Habitat Hydraulics and Upstream Migration of Neritina virginea (Gastropoda: Neritidae) in Northeastern Puerto Rico

BLANCO, JUAN F.; SCATENA, FREDERICK N. 2005. Floods, Habitat Hydraulics and Upstream Migration of Neritina virginea (Gastropoda: Neritidae) in Northeastern Puerto Rico.. Caribbean Journal of Science, Vol. 41, No. 1, 55-74, .

Massive upstream migrations of neritid snails (Neritidae: Gastropoda) occur in tropical and subtropical streams worldwide, but their seasonality and proximate causes are unknown. We monitored massive upstream migrations of Neritina virginea for 99 weeks, and conducted a detailed study of snail density, size, and hydraulic descriptors in lower Río Mameyes, northeastern Puerto Rico. The study assessed the 1) timing and seasonality of upstream migration, 2) size composition of migratory aggregations, 3) patterns of habitat use, and 4) role of floods on upstream migration. Massive upstream migrations (500–3000ind/m2) were observed in 44 of 99 weeks of observation. While N. virginea aggregations occurred at random time intervals, they were clumped during rainy periods. Migratory aggregations consisted mostly of small individuals (5-7 mm). Greater mean density was consistently observed in a stable riffle than in an unstable run (115.7 and 17.8 ind/m2, respectively), but mean density increased and mean size reduced in both reaches during the first 7 upstream migratory events. N. virginea density and size dynamics differed between reaches as a function of habitat hydraulics. While juveniles used the stable riffle as a permanent habitat and preferred passageway, they also used an adjacent, unstable reach after storm events. Density variation was correlated with days postflood (>3.5 m3/s) in both reaches. Our observations indicated that massive upstream migrations of N. virginea juveniles occur at least once a month, presumably as habitat-dependent responses to floods.

Hierarchical contribution of river–ocean connectivity, water chemistry, hydraulics, and substrate to the distribution of diadromous snails in Puerto Rican streams

Blanco, Juan F.; Scatena, Frederick N. 2006. Hierarchical contribution of river-ocean connectivity, water chemistry, hydraulics, and substrate to the distribution of diadromous snails in Puerto Rican streams.. J. N. Am. Benthol. Soc., 25(1) :82-98.

Diadromous faunas dominate most tropical coastal streams and rivers, but the factors controlling their distribution are not well understood. Our study documents abiotic variables controlling the distribution and abundance of the diadromous snail Neritina virginea (Gastropoda:Neritidae) in the Caribbean island of Puerto Rico. An intensive survey of N. virginea density and shell size, and channel substrate, velocity, and depth was conducted at microhabitat, habitat, and reach scales of a coastal plain reach of the Río Mameyes between August and December 2000. In addition, the inland extent of distribution (stream-network scale) and presence (regional scale) of N. virginea were surveyed in 32 coastal rivers around the island during summer 2001 and 2003. At the microhabitat scale, snail density and microhabitat electivity were greater in patches consisting of a mix of boulders and cobbles than in other types of substrate. At the habitat scale, snail density increased with depth. At the reach scale, snail density increased with fast and turbulent flows (riffle > pools > pond), whereas snail size showed the opposite pattern. At the regional scale, populations were present in 13 of 32 streams. Populations of N. virginea were not found in rivers that were disconnected from the ocean for most of the year because of channel dewatering, formation of sediment bars at their mouths, and low mean monthly discharge (Q=0.69 m3/s). In contrast, rivers with N. virginea populations had a permanent (Q=4.04 m3/s) or seasonal (Q=2.88 m3/s) connection to the ocean over the year. At the regional scale, the inland distribution of populations was not correlated with stream gradient, but was negatively correlated with concentrations of SiO2, P, and acid neutralizing capacity of the water. Populations colonized montane reaches in only 5 rivers, all of which were forested and protected. Our study highlights the importance of taking a hierarchical approach in managing tropical coastal rivers, and the usefulness of neritid snails as biological indicators of the physical and chemical integrity of rivers.
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