DOI: 10.5061/dryad.2280gb5t6
title: Reproductive benefits associated with dispersal in headwater populations of Trinidadian guppies (Poecilia reticulata)

description: Theory suggests that the evolution of dispersal is balanced by its fitness
 costs and benefits, yet empirical evidence is sparse due to the
 difficulties of measuring dispersal and fitness in natural populations.
 Here, we use spatially-explicit data from a multi-generational
 capture-mark-recapture study of two populations of Trinidadian guppies
 (Poecilia reticulata) along with pedigrees to test whether there are
 fitness benefits correlated with dispersal. Combining these ecological and
 molecular datasets allows us to directly measure the relationship between
 movement and reproduction. Individual dispersal was measured as the total
 distance moved by a fish during its lifetime. We analyzed the effects of
 dispersal propensity and distance on a variety of reproductive metrics. We
 found that number of mates and number of offspring produced were
 positively correlated to dispersal, especially for males. Our results also
 reveal individual and environmental variation in dispersal, with sex,
 size, season, and stream acting as determining factors. 
description: <strong>Field
 capture-mark-recapture</strong><br> We studied low predation
 populations of <em>P. reticulata</em> in neighboring streams,
 Taylor and Caigual, in the Guanapo drainage on the south slope of the
 Northern Range of Trinidad. Data used for this project were collected in a
 spatially explicit, monthly capture-mark-recapture study that spanned June
 2009-July 2011. In April 2009, as part of a separate study (see Travis et
 al. 2014), guppies from downstream site within the same drainage were
 translocated upstream of our two focal sites. Translocated individuals
 eventually reached and bred with the focal resident populations
 (Fitzpatrick et al., 2016, 2020). However, our goal for this study was to
 focus on dispersal behavior of the resident population. Therefore,
 individuals reported on in this study were fish captured in the first 13
 months of the mark-recapture study (June 2009-July 2010), when immigration
 of translocated individuals into the focal sites was minimal. To account
 for the presence of few immigrant and hybrids in our dataset, we included
 a hybrid index covariate in all statistical models. This hybrid index
 varied from 0 (pure resident) to 1 (pure immigrant) and was calculated
 using genetic data, as described below (see Fitzpatrick et al. 2020). Only
 5% of fish in our dataset were classified as pure immigrants, suggesting
 that the vast majority of our dataset represents the resident populations
 of Taylor and Caigual prior to the onset of gene flow (Fitzpatrick et al.,
 2016). Detailed capture-mark-recapture methods are
 described in Fitzpatrick et al. (2016). Briefly, we selected a portion of
 each stream that was the upstream-most extent of wild guppy populations
 uninterrupted by waterfall barriers. The sampled reach in Taylor was 240 m
 in length, and 80 m in length in Caigual. Each distinct pool or riffle
 within focal reaches was uniquely labelled and sampled monthly using a
 combination of hand nets and mesh traps. All guppies greater than 14 mm
 were captured, transferred to the lab, and placed in aerated tanks,
 separated by pool location and sex. Only mature individuals were included
 in the analyses, as determined by gonopodium morphology for males, and the
 presence of melanophores at the cloaca for females. These methods are
 commonly used in capture-mark-recapture studies of Trinidadian guppies,
 and have been shown to produce low lab mortality and high capture
 probabilities (Reznick et al. 1996). During lab processing, individuals
 were anesthetized with a dilute solution of MS-222, and new recruits were
 given a unique subcutaneous elastomer mark (Northwest Marine Technologies,
 Inc., Shaw Island, WA, USA). Recruits had three scales collected and dried
 for DNA extraction, and all individuals were weighed and photographed each
 month. All fish were returned to their exact capture location one to two
 days after processing. During their release, fish were acclimated to
 stream water and released into the lowest flow region of their capture
 location to minimize accidental passive downstream movement.  
 <strong>Quantifying dispersal
 variation</strong><br> The exact pool locations of initial
 capture and subsequent recaptures were recorded every sampling event for
 all individuals. Although sizes and locations of pools and riffles change
 based on seasonal dynamics, they are always noted by the number of meters
 from the upstream extent of the reach. <em>P. reticulata</em>
 were considered philopatric (non-dispersing) if they were consistently
 captured less than 10 m from their initial capture location, the
 approximate length of the maximum pool size. Individuals were considered
 dispersers if at any point in the study they were captured 10 m or more
 from the pool where they were first captured, regardless of how many
 sampling occasions it took for that movement to occur. 
 We use the threshold of 10 m because that is the length of the
 largest pool in our study, such that any movement beyond it reflects that
 an individual departed its initial pool and settled at a new location for
 a period of time. Given that most guppy daily activity takes place at the
 within-pool scale and movement between pools places an individual in a
 new, non-local environment, this threshold matches our definition of
 dispersal as “a departure from a local environment, followed by movement
 and eventual settlement in a new location.” A potential source of error in
 our estimate of distance could stem from individuals moving and
 backtracking between sampling occasions, such that total movement would be
 underestimated. Thus, we are only able to estimate minimum distance
 moved.  In addition to the categorical classification
 of philopatric or dispersing, we quantified the total distance travelled
 for all individuals as the cumulative distance the moved during our study
 (Figure 1). This estimate considers upstream and downstream movement
 equally. For example, a fish that moved upstream from 0 m to 10 m to 20 m
 would have the same dispersal distance (10 + 10 = 20 m) as a fish that
 moved upstream from 0 m to 10 m, then back downstream from 10 m to 0 m (10
 – 0 + 10 = 20 m). Finally, we calculated the range of all dispersing fish,
 defined as the minimum non-cumulative distance spanning all of its
 locations across the study––in the example above, the first fish has a
 range of 20 m, but the second fish has a range of 10 m.