Date of Award


Degree Type


Degree Name

Doctor of Philosophy in Biological and Environmental Sciences


Fisheries, Animal and Veterinary Science

First Advisor

Austin Humphries


The deep-slope demersal fishery in Indonesia is a deep-water (50-500 m) fishery targeting multiple snapper (Lutjanidae), grouper (Epinephelidae), croaker (Scianidae), jack and trevally (Carangidae), and emperor (Lethrinidae) species. Many target species have slow growth rates, later age at maturity, and extended longevity making them particularly vulnerable to over-exploitation. This fishery is economically important and positions Indonesia as the second-largest snapper exporter globally. Fishers utilize dropline, longline, mix-gears (dropline and longline concurrently), traps, and gillnets, depending on the fishing grounds’ habitats. However, the main fishing gears are droplines and longlines. Vessel sizes vary from 1 gross ton (GT) to 150 GT, depending on the fishing gear and fishing grounds. Exploitation for the deep-slope demersal fishery is open-access with no regulation.

To maintain the sustainability of the fishery, fishery managers must implement effective policies. The efficacy of fisheries management, however, depends on the regulation’s applicability with the characteristics of the resources (catch) and of the resource-users (fishers). Basic information on the fishery, such as exploitation levels and biological information of the catch (i.e., life history characteristics), is necessary. Using data from the Crew-Operated Data Recording System (CODRS) developed by The Nature Conservancy Indonesia (Yayasan Konservasi Alam Nusantara), we aim to characterize the fishery, assess the fishery using multiple fishery indicators, and provide potential management options.

Chapter 1 describes the characteristics of the fishery and catch species. Chapter 1 also covers different fishery indicators that are frequently used in data-poor fisheries. The information provided in Chapter 1 serves as background information to Chapters 2, 3, and 4. In Chapter 2, we validated the CODRS as a viable collaborative data collection method by comparing the total catch weight from CODRS with that of trade receipts. Furthermore, we characterized the fishery by identifying top species caught by each fishing gear and mapped the fleet across the entire Indonesian archipelago. Length-based stock assessments using the updated life-history parameters showed high risks of overfishing (i.e., low Spawning Potential Ratio (SPR), a high percentage of immatures in the catch) for most of the primary target species, especially snappers with large maximum sizes.

In Chapter 3, we investigated different factors that influence the proportion of immature fish in the catch. We focused on six main target species: Pristipomoides multidens, Pristipomoides filamentosus, Pristipomoides typus, Atrobucca brevis, Epinephelus areolatus, and Lutjanus malabaricus. Results indicated that immature fish assemblages were caught in particular locations, or ‘hotspots,’ through a combination of fishing gears and habitat types. The primary ‘hotspots’ occurred in the Java Sea-Makassar Strait area. When comparing hotspots and preexisting Marine Protected Areas (MPA), only 2.4% of MPA were located within ‘hotspots’. These findings highlight places of high conservation priorities, such as the Java Sea, where the expansion of current MPAs would greatly benefit the deep-slope demersal fishery in Indonesia by reducing immature catches.

In Chapter 4, we standardized the catch per unit effort (CPUE) of five speciesfishing gear complexes: L. malabaricus-dropline, L. malabaricus-longline, P. multidensdropline, P. multidens-longline, and A. rutilans-dropline. Although there is not enough time-series data to calculate a positive or negative CPUE trend, the comparison between standardized CPUE and SPR showed a decoupling between the two fishery indicators. Several fishing grounds (i.e., in the Java sea, Makassar Strait, and Arafura Sea) have high CPUE but low to moderate SPR (<40 %). The disjunction indicated possible hyperstability of the CPUE index. Thus, these fishing grounds in the Java Sea, Makassar Strait, and the Arafura Sea may be conservation priority areas. Also, the seemingly conflicting findings between different indicators illustrate the importance of using more than one fishery indicator to get a holistic view of the fishery.

In Chapter 5, we summarized the previous chapters' findings, identified the limitations in the analysis, and provided directions for the next steps to formulate fishery management strategies for the deep-slope demersal fishery.

Available for download on Friday, December 16, 2022