Final Project: San Diego Bottlenose Dolphin Sighting Distributions
The Research Question:
Originally, I asked the question: do common bottlenose dolphin sighting distances from shore change over time?
However, throughout the research and analysis process, I refined this question for a multitude of reasons. For example, I planned on using all of my dolphin sightings from my six different survey locations along the California coastline. Because the bulk of my sightings are from the San Diego survey site, I chose this data set for completeness and feasibility. Additionally, this data set used the most standard survey methods. Rather than simply looking at distance from shore, which would be at a very fine scale, seeing as all of my sightings are within two kilometers from shore, I chose to try and identify changes in latitude. Furthermore, I wanted to see if changes in latitude (if present, were somehow related to the El Nino Southern Oscillation (ENSO) cycles and then distances to lagoons). This data set also has the largest span of sightings by both year and month. When you see my hypotheses, you will notice that my original research question morphed into much more specific hypotheses.
Data Description:
My dolphin sighting data spans 1981-2015 with a few absent years, and sightings covering all months, but not in all years sampled. The same transects were performed in a small boat with approximately a two kilometer sighting span (one kilometer surveyed 90 degrees to starboard and port of the bow). These data points therefore have a resolution of approximately two kilometers. Much of the other data has a coarser spatial resolution, which is why it was important to use such a robust data set. The ENSO data I used gave a broad brushstroke approach to ENSO indices. Rather than first using the exact ENSO index which is at a fine scale, I used the NOAA database that split month-years into positive, neutral, and negative indices (1, 0, and -1, respectively). These data were at a month-year temporal resolution, which I matched to my month-date information of my sighting data. Lagoon data were sourced from the mid-late 2000s, therefore I treated lagoon distances as static.
Hypotheses:
H1: I predicted that bottlenose dolphin sightings at the pod-scale (usually, one to ten individuals) along the San Diego transect throughout the years 1981-2015 would exhibit clustered distribution patterns as a result of the patchy distributions of both the species’ preferred habitats and prey, as well as the social nature of this species.
H2: I predicted there would be higher densities of bottlenose dolphin sightings at the pod-scale (usually, one to ten individuals) at higher latitudes spanning 1981-2015 due to prey distributions shifting northward and less human activities in the northward sections of the transect. I predicted that during warm (positive) ENSO months, the dolphin sightings in San Diego would be distributed more northerly, predominantly with prey aggregations historically shifting northward into cooler waters, due to (secondarily) increasing sea surface temperatures. I expect the spatial gradient to shift north and south, in relation to the ENSO gradient (warm, neutral, or cold)
H3: I predicted that along the San Diego coastline, bottlenose dolphin sightings at the pod-scale (usually, one to ten individuals) would be clustered around the six major lagoons within about two kilometers, with no specific preference for any lagoon, because the murky, nutrient-rich waters in the estuarine environments are ideal for prey protection and known for their higher densities of schooling fishes.
Map with bottlenose dolphin sightings on the one-kilometer buffered transect line and the six major lagoons in San Diego.
Approaches:
I utilized multiple approaches with different software platforms including ArcMap, qGIS, GoogleEarth, and R Studio (with some Excel data cleaning).
- Buffers in ArcMap
- Calculations in an attribute table
- ANOVA with Tukey HSD
- Nearest Neighbor averages
- Cluster analyses
- Histograms and Bar plots
Results:
I produced a few maps (will be), found statistical relationships between sightings and distribution patterns, ENSO and dolphin latitudes, and distances to lagoons.
H1: I predicted that bottlenose dolphin sightings at the pod-scale (usually, one to ten individuals) along the San Diego transect throughout the years 1981-2015 would exhibit clustered distribution patterns as a result of the patchy distributions of both the species’ preferred habitats and prey, as well as the social nature of this species.
True: The results of the average nearest neighbor spatial analysis in ArcMap 10.6 produced a z-score of -127.16 with a p-value of < 0.000001, which translates into there being less than a 1% likelihood that this clustered pattern could be the result of random chance. Although I could not look directly at prey distributions because of data availability, it is well-known that schooling fishes exist in clustered distributions that could be related to these dolphin sightings also being clustered. In addition, bottlenose dolphins are highly social and although pods change in composition of individuals, the dolphins do usually transit, feed, and socialize in small groups. Also see Exercise 2 for other, relevant preliminary results, including a histogram of the distribution in differences of sighting latitudes.
Summary from the Average Nearest Neighbor calculation in ArcMap 10.6 displaying that bottlenose dolphin sightings in San Diego are highly clustered.
H2: I predicted there would be higher densities of bottlenose dolphin sightings at the pod-scale (usually, one to ten individuals) at higher latitudes spanning 1981-2015 due to prey distributions shifting northward and less human activities in the northward sections of the transect. With this, I predicted that during warm (positive) ENSO months, the dolphin sightings in San Diego would be distributed more northerly, predominantly with prey aggregations historically shifting northward into cooler waters, due to (secondarily) increasing sea surface temperatures. I expect the spatial gradient to shift north and south, in relation to the ENSO gradient (warm, neutral, or cold).
False: the sightings are more clumped towards the lower latitudes overall (p < 2e-16), possibly due to habitat preference. The sightings are closer to beaches with higher human densities and human-related activities near Mission Bay, CA. It should be noted, that just north of the San Diego transect is the Camp Pendleton Marine Base which conducts frequent military exercises and could deter animals.
I used an ANOVA analysis and found there was a significant difference in sighting latitude distributions between monthly ENSO indices. A Tukey HSD was performed to determine where the differences between treatment(s) were significant. All differences (neutral and negative, positive and negative, and positive and neutral ENSO indices) were significant with p < 0.005.
H3: I predicted that along the San Diego coastline, bottlenose dolphin sightings at the pod-scale (usually, one to ten individuals) would be clustered around the six major lagoons within about two kilometers, with no specific preference for any lagoon, because the murky, nutrient-rich waters in the estuarine environments are ideal for prey protection and known for their higher densities of schooling fishes. See my Exercise 3 results.
Using a histogram, I was able to visualize how distances to each lagoon differed by lagoon. That is dolphin sightings nearest to, Lagoon 6, the San Dieguito Lagoon, are always within 0.03 decimal degrees. In comparison, Lagoon 5, Los Penasquitos Lagoon, is distributed across distances, with the most sightings at a great distance.
Bar plot displaying the different distances from dolphin sighting location to the nearest lagoon in San Diego in decimal degrees. Note: Lagoon 4 is south of the study site and therefore was never the nearest lagoon.
After running an ANOVA in R Studio, I found that there was a significant difference between distance to nearest lagoon in different ENSO index categories (p < 2.55e-9) with a Tukey HSD confirming that the significant difference in distance to nearest lagoon being significant between neutral and negative values and positive and neutral years. Therefore, I gather there must be something happening in neutral months that changes the distance to the nearest lagoon, potentially prey are more static or more dynamic in those years compared to the positive and negative months. Using a violin plot, it appears that Lagoon 5, Los Penasquitos Lagoon, has the widest span of sighting distances when it is the nearest lagoon in all ENSO index month values. In neutral years, Lagoon 0, the Buena Vista Lagoon has more than a single sighting (there were none in negative months and only one in positive months). The Buena Vista Lagoon is the most northerly lagoon, which may indicate that in neutral ENSO months, dolphin pods are more northerly in their distribution.
Takeaways to science and management:
Bottlenose dolphins have a clustered distribution which seems to be related to ENSO monthly indices, with certain years having more of a difference in distribution, and likely, their sociality on a larger scale. Neutral ENSO months seem to have a different characteristic that impact sighting distribution locations along the San Diego coastline. More research needs to be done in this to determine what is different about neutral months and how this may impact this dolphin population. On a finer scale, the six lagoons in San Diego appear to have a spatial relationship with dolphin pod sighting distributions. These lagoons may provide critical habitat for bottlenose dolphin preferred prey species or preferred habitat for the dolphins themselves either for cover or for hunting, and different lagoons may have different spans of impact at different distances, either by creating larger nutrient plumes, or because of static, geographic and geologic features. This could mean that specific areas should be protected more or maintain protection. For example, the Batiquitos and San Dieguito Lagoons have some Marine Conservation Areas with No-Take Zones. It is interesting to see the relationship to different lagoons, which may provide nutrient outflows and protection for key bottlenose dolphin prey species. The city of San Diego and the state of California are need ways to assess the coastlines and how protecting the marine, estuarine, and terrestrial environments near and encompassing the coastlines impact the greater ecosystem. Other than the Marine Mammal Protection Act and small protected zones, there are no safeguards for these dolphins.
My Learning: about software (a) Arc-Info and b) R
- a) Arc-Info: buffer creation, creating graphs, nearest neighbor analyses. How to deal with transects, certain data with mismatching information, conflicting shapefiles
- b) R: I didn’t know much, except the basics in R. I learned about how to conduct ANOVAs and then how to interpret results. Mainly I learned about how to visualize my results and use new packages.
My Learning: about statistics
Throughout this project I learned that spatial statistics requires clear hypothesis testing in order to clearly step through a spatial process. Most specifically, I learned about spatial analyses in ArcMap, and how I could utilize nearest neighbor calculations to assess distribution patters. Furthermore, I now have a better understanding of spatial distribution patterns and how they are assessed, such as clustering versus random versus equally dispersed distributions. For more data analysis and cleaning, I also learned how to apply my novice understanding of ANOVAs and then display results relating to spatial relationships (distances) using histograms and other graphical displays in R Studio.
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Illustrating the number of sightings grouped by ENSO index values (1, 0, and -1).