Aside from the 15 PRC vessels, the satellite image also shows the PCG's 44-meter BRP Sindangan anchored just off Thitu Island, having escorted two Bureau of Fisheries and Aquatic Resources (BFAR) vessels, as reported overnight by GMA news. 

The Philippine vessels arrived on 19 March, together with second PCG ship, the 44-meter BRP Cabra. The Cabra, however, then departed Thitu the following day and was most recently detected patrolling near Sabina Shoal.

According to the GMA report, the joint PCG-BFAR mission to Thitu--which the Philippines calls Pag-Asa--is supporting an environmental impact study:

The BFAR and marine researchers from the University of the Philippines officially began their joint maritime resource assessment to identify the current state of the Pag-asa Cays following the discovery of dead corals in Sandy Cay Two last year, which were believed to have come from another area.

Experts also said that they planned to study whether the dumping of dead and crushed corals at Sandy Cay Two was done on purpose.

The PRC vessel swarm includes the 100-meter China Coast Guard (CCG) 5204, which was just 1.5 nautical miles from the island. The CCG ship is running "dark", which means it is not broadcasting a detectable automatic information system (AIS) signal as required by International Maritime Organization (IMO) regulations. However, GMA's camera's clearly show the ship's identity.

The PRC militia ships off Thitu include nine of the large Qiong Sansha Yu type, purpose-built 55-60 meter vessels with professional crews trained to take part in direct military and paramilitary action, such as the blockade of Second Thomas Shoal or enforcement of China's claimed jurisdiction over Scarborough Shoal.

Under the United Nations Convention on the Law of the Sea, Thitu Island earns a 12-nautical mile territorial sea, which would make the PRC vessels' presence illegal. Beijing, however, claims Thitu as its own sovereign territory, so these ships' presence signals China's direct challenge to the Philippines' 53-year occupation of the island.

The remainder of the ships are likely Spratly Backbone Vessels (SBV), smaller vessels which receive government subsidies to help assert PRC sovereignty through presence operations. Two of these ships can be seen rafted together in the satellite image near the northern cays. These SBVs generally broadcast Class-B AIS signals, which are not detectable by satellite and thus not visible on the image above.

The GMA report also notes that during their 19 March voyage to Thitu Island, the PCG and BFAR vessels were shadowed by a PRC Navy vessel, the Type 54A frigate Xuzhou (hull number 530).

The BRP Cabra's ongoing patrol near Sabina Shoal, meanwhile, seems to have attracted the PRC's attention, as nine Qiong Sansha Yu millitia ships have redeployed from the west to Mischief Reef, the artificial island and military base from which China stages its Second Thomas Shoal blockade force. Presumably these ships are preparing to redeploy further east of Second Thomas Shoal, should China believe the BRP Cabra is preparing to join up with rotation and resupply (RoRe) vessels and proceed eastward to the Philippine outpost aboard the rusted BRP Sierra Madre.

Whether or not the Philippines plans a RoRe mission for this voyage remains to be seen. Sabina Shoal is also where China has recently deployed at least 26 Spratly Backbone Vessels, so the BRP Cabra's patrol there may simply be intended to investigate that development.

UPDATE: China claims to have "thwarted" an effort by 34 Philippine nationals to access Sandy Cay (Chinese, Tiexian Jiao), which is the northernmost above-water feature in the Pag-Asa Cays. In the Planet image above it is just north of the China Coast Guard ship 5204, about 1.5 nautical miles northwest of Thitu Island. 

It seems likely the 34 individuals in question were there in support of the environmental impact study referred to above. 

From the PRC's state-controlled media outlet CGTN: