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Book Review

Leaving the nursery: fish migration between juvenile and adult habitats

Paper: Huijbers, C. M., I. Nagelkerken, and C. A. Layman. 2015. Fish movement from nursery bays to coral reefs: a matter of size? Hydrobiologia 750:89-101. DOI 10.1007/s10750-014-2162-4

Introduction:

Marine habitats are connected in many ways, such as through the organisms that move between them.Many species utilize multiple habitats to forage, rest, and spawn, for example. Some species undertake ontogenetic habitat shifts (simply meaning they use different habitats as juveniles and as adults). Such habitat shifts require special consideration when designing management programs. To design an effective Marine Protected Area for a species that undertakes such a shift, it is critical to understand all areas utilized within each habitat and patterns of movement within and between them. It is important to answer how much of each habitat does an individual utilize? When do they move between areas? How does this migration occur—with a sudden movement from one habitat to the other or in a stepwise fashion. Huijbers et al. used state-of-the-art technology to try to answer these very questions!

Fig. 1. Adult Schoolmaster Snappers (Lutjanus apodus); Source: Florent Charpin, http://reefguide.org/pixhtml/schoolmaster2.html

Fig. 1. Adult Schoolmaster Snappers (Lutjanus apodus); Source: Florent Charpin, http://reefguide.org/pixhtml/schoolmaster2.html

Methods:

The researchers chose a focal species known to exhibit ontogenetic habitat shifts, the schoolmaster snapper (Lutjanus apodus) (Fig. 1). Juveniles tend to be found in mangrove habitats and adults on reefs. Snappers were tracked using acoustic tags and a network of receivers. Seventy-two fish were tagged, each with a unique acoustic pulse signature allowing identification of individual fish. Fifteen receivers were arrayed in a network: eight in the channel—primarily mangrove and seagrass habitat—, one in the mouth of the bay, and six on the coral reef (Fig. 2). These receivers record the date, time and individual signature of any tagged fish that passes within range and “pings” the receiver with its transmitter.

Fig. 2. The array of receiver networks in the channel (#1-8), mouth of the bay (#9) and on the reef (#10-15) that allowed tracking of tagged fish.

Fig. 2. The array of receiver networks in the channel (#1-8), mouth of the bay (#9) and on the reef (#10-15) that allowed tracking of tagged fish.

Results and Conclusions:

Over the course of one year, the network of receivers picked up 341,342 tag detections! From so much data, interesting trends and patterns were revealed. For example, most fish were detected within 500 m from their “home” receiver—the one nearest their daytime shelter. The schoolmaster snappers were most active at nighttime, with almost double the detections at night than between 12 and 7 pm.

The most interesting (to me at least!) focused on seven fish (out of those 72 tagged) that displayed movement between mangrove and seagrass habitats in the bay and the coral reefs (Fig. 3). These seven were larger than others in the study. Five of them were detected at the mouth of the bay between midnight and 6 am, so the authors suggest these excursions likely occur at night. Three fish were categorized as visitors to the reef since they pinged “home” both before and after excursions to the mouth of the bay (and likely the reef beyond). One visitor was detected visiting the reef three times with about one month between each occasion. The other four were classified as having permanently moved to the reef. Two passed the mouth of the bay to leave but never returned. They were never detected on the reef, so may have moved permanently (or been eaten?) The other two were both detected on the reef, about 1 km from the mouth of the bay.

Fig. 3. Summary of the movements of the 7 fish that moved toward the reef. The left image shows the three that visited the reef (pinged in the bay both before and after pinging on the reef or at station 9—mouth of bay). The right image shows the movements of the four that permanently moved to the reef (no pings at receivers in the bay after detection on the reef or at the mouth of the bay).

Fig. 3. Summary of the movements of the 7 fish that moved toward the reef. The left image shows the three that visited the reef (pinged in the bay both before and after pinging on the reef or at station 9—mouth of bay). The right image shows the movements of the four that permanently moved to the reef (no pings at receivers in the bay after detection on the reef or at the mouth of the bay).

Challenges:

There are many challenges involved when studying species’ movement, especially for infrequent ones, such as ontogenetic habitat shifts. The authors did highlight that only larger fish made the bay to reef movement, possibly due to risk of predation, need for larger prey, or reaching reproductive maturity. With only 7 of 72 fish making this movement in the course of a year, it is difficult to make conclusions about how these fish are moving between habitats. Longer monitoring studies (and increased battery life for tags) or those directed at fish of the size that are likely preparing to make that shift may be needed. Finally, using acoustic tags limits what information can be gathered once a tag stops transmitting—the transmitter may have failed, the fish may have died or it may have moved outside the zone equipped with receivers. The researcher may never know! These challenges are unfortunate but highlight the realistic limitations of research.

Importance:

Knowing how coastal fish move between habitats at different life stages is important when considering management—choosing what areas to protect, how large a region, etc. These researchers found that movements from sheltered nursery habitats to coral reefs may be either more like a leap of faith or a series of exploratory missions. Therefore, depending on your species of interest and the life stage at which they are most vulnerable, you might have to protect the nursery, migration path, and the adult habitat. Their findings, like so many in science, highlighted the need for further research but provided one more piece to the puzzle.

Tell me what you think!: Along yesterday’s theme: if you could tag animals to learn about a specific type of behavior, what behavior would you want to target?

Discussion

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