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. 

Field capture-mark-recapture
We studied low predation populations of P. reticulata 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.  

Quantifying dispersal variation
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. P. reticulata 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.