Altered juvenile fish communities associated with invasive <i>Halophila stipulacea</i> seagrass habitats in the U.S. Virgin Islands

<div><p>Caribbean seagrass habitats provide food and protection for reef-associated juvenile fish. The invasive seagrass <i>Halophila stipulacea</i> is rapidly altering these seascapes. Since its arrival in the Caribbean in 2002, <i>H</i>. <i>stipulacea</i> has colonized and displaced native seagrasses, but the function of this invasive seagrass as a juvenile fish habitat remains unknown. To compare diversity, community structure, and abundance of juvenile fish between <i>H</i>. <i>stipulacea</i> and native seagrass beds, fish traps were deployed in four nearshore bays around St. Thomas, U.S. Virgin Islands. Traps were deployed in Frenchman, Lindbergh, and Sprat Bays for 24 h intervals in patches of bare sand, patches of <i>H</i>. <i>stipulacea</i> and patches of the native Caribbean seagrasses <i>Thalassia testudinum</i> and S<i>yringodium filiforme</i>. Traps were then deployed in Brewers Bay for 12 h intervals in stands of <i>H</i>. <i>stipulacea</i> and <i>S</i>. <i>filiforme</i>. Relative and total abundances of juvenile fish, identified at least to family, were compared across treatment habitats for each trap deployment period. The catch from <i>H</i>. <i>stipulacea</i>, compared to native seagrasses, comprised a greater abundance of nocturnal carnivores <i>Lutjanus synagris</i> (family Lutjanidae) and <i>Haemulon flavolineatum</i> (family Haemulidae). Additionally, the herbivore species <i>Sparisoma aurofrenatum</i> (family Labridae) and <i>Acanthurus bahianus</i> (family Acanthuridae) and the diurnal carnivore species <i>Pseudopeneus maculatus</i> (family Mullidae) were relatively scarce in <i>H</i>. <i>stipulacea</i>. The catch from sand was much smaller, compared to vegetated habitats, and comprised only <i>L</i>. <i>synagris</i>, <i>H</i>. <i>flavolineatum</i>, and <i>H</i>. <i>aurolineatum</i>. These results provide evidence of reduced family diversity and altered juvenile fish assemblages in <i>H</i>. <i>stipulacea</i>, driven by an abundance of some nocturnal carnivores and scarcity of herbivores and diurnal carnivores. The findings from the present work underpin the need for further investigation and mitigation of this invasion, particularly where <i>H</i>. <i>stipulacea</i> is driving seascape-alterations of key juvenile fish habitats.</p></div

NMDS of juvenile fish family relative abundance during 12 h deployments.

<p>Points represent individual traps, and 95% confidence ellipses indicate sampling distributions for each seagrass. Data were pooled across soak types.</p

Negative binomial generalized linear model results for each guild during 12 h deployments.

<p>Negative binomial generalized linear model results for each guild during 12 h deployments.</p

Mean juvenile fish abundance in 12 h trap deployments.

<p>(A) Mean abundance of the total catch (<b>±</b> SEM), separated by treatment habitat and soak type. The contribution of each family to the mean abundance is indicated by stacked colored bars. For each guild, a legend with colors corresponding to each family and plots of the mean abundance (<b>± SEM)</b> across seagrass habitats are given for (B) nocturnal carnivores, (C) diurnal carnivores, and (D) herbivores.</p

Mean juvenile fish abundance (± SEM) in each trap during 24 h trap deployments.

<p>Separated by treatment habitat (<i>H</i>. <i>stipulacea</i>, <i>S</i>. <i>filiforme</i>, <i>T</i>. <i>testudinum</i>, and sand) and site (F = Frenchman Bay, L = Lindbergh Bay, and S = Sprat Bay). The contribution of each family to the mean abundance is indicated by stacked colored bars. Letters from a Tukey HSD test indicate significant differences across sites and habitats.</p

NMDS of each family’s relative abundance across benthic habitats during 24 h deployments.

<p>Points represent individual traps, and 95% confidence ellipses indicate sampling distributions for each habitat. Data were pooled across site.</p

Two-way analyses of variance (ANOVAs).

<p>Two-way analyses of variance (ANOVAs).</p

Average percent cover and canopy height of seagrass patches used as trap deployment locations.

<p>Average percent cover and canopy height of seagrass patches used as trap deployment locations.</p

Lengths at maturity for species identified in 24 and 12 h deployments.

<p>Lengths at maturity for species identified in 24 and 12 h deployments.</p

Total catch of juvenile fish in 24 and 12 h trap deployments.

<p>Total catch of juvenile fish in 24 and 12 h trap deployments.</p